diff --git a/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/get_custom_metric_label.ts b/x-pack/plugins/infra/common/formatters/get_custom_metric_label.ts
similarity index 91%
rename from x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/get_custom_metric_label.ts
rename to x-pack/plugins/infra/common/formatters/get_custom_metric_label.ts
index 495cc8197d2e78..3be5986d489d32 100644
--- a/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/get_custom_metric_label.ts
+++ b/x-pack/plugins/infra/common/formatters/get_custom_metric_label.ts
@@ -5,7 +5,7 @@
  */
 
 import { i18n } from '@kbn/i18n';
-import { SnapshotCustomMetricInput } from '../../../../../../../common/http_api/snapshot_api';
+import { SnapshotCustomMetricInput } from '../http_api/snapshot_api';
 
 export const getCustomMetricLabel = (metric: SnapshotCustomMetricInput) => {
   const METRIC_LABELS = {
diff --git a/x-pack/plugins/infra/public/alerting/inventory/components/alert_flyout.tsx b/x-pack/plugins/infra/public/alerting/inventory/components/alert_flyout.tsx
index bb08c5b85b8539..79dff1b0be00c5 100644
--- a/x-pack/plugins/infra/public/alerting/inventory/components/alert_flyout.tsx
+++ b/x-pack/plugins/infra/public/alerting/inventory/components/alert_flyout.tsx
@@ -12,6 +12,7 @@ import { useKibana } from '../../../../../../../src/plugins/kibana_react/public'
 import { METRIC_INVENTORY_THRESHOLD_ALERT_TYPE_ID } from '../../../../server/lib/alerting/inventory_metric_threshold/types';
 import { InfraWaffleMapOptions } from '../../../lib/lib';
 import { InventoryItemType } from '../../../../common/inventory_models/types';
+import { useAlertPrefillContext } from '../../../alerting/use_alert_prefill';
 
 interface Props {
   visible?: boolean;
@@ -21,16 +22,24 @@ interface Props {
   setVisible: React.Dispatch<React.SetStateAction<boolean>>;
 }
 
-export const AlertFlyout = (props: Props) => {
+export const AlertFlyout = ({ options, nodeType, filter, visible, setVisible }: Props) => {
   const { triggersActionsUI } = useContext(TriggerActionsContext);
   const { services } = useKibana();
 
+  const { inventoryPrefill } = useAlertPrefillContext();
+  const { customMetrics } = inventoryPrefill;
+
   return (
     <>
       {triggersActionsUI && (
         <AlertsContextProvider
           value={{
-            metadata: { options: props.options, nodeType: props.nodeType, filter: props.filter },
+            metadata: {
+              options,
+              nodeType,
+              filter,
+              customMetrics,
+            },
             toastNotifications: services.notifications?.toasts,
             http: services.http,
             capabilities: services.application.capabilities,
@@ -40,8 +49,8 @@ export const AlertFlyout = (props: Props) => {
           }}
         >
           <AlertAdd
-            addFlyoutVisible={props.visible!}
-            setAddFlyoutVisibility={props.setVisible}
+            addFlyoutVisible={visible!}
+            setAddFlyoutVisibility={setVisible}
             alertTypeId={METRIC_INVENTORY_THRESHOLD_ALERT_TYPE_ID}
             canChangeTrigger={false}
             consumer={'infrastructure'}
diff --git a/x-pack/plugins/infra/public/alerting/inventory/components/expression.test.tsx b/x-pack/plugins/infra/public/alerting/inventory/components/expression.test.tsx
new file mode 100644
index 00000000000000..ada7a30a859e08
--- /dev/null
+++ b/x-pack/plugins/infra/public/alerting/inventory/components/expression.test.tsx
@@ -0,0 +1,186 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+import { mountWithIntl, nextTick } from 'test_utils/enzyme_helpers';
+import { actionTypeRegistryMock } from '../../../../../triggers_actions_ui/public/application/action_type_registry.mock';
+import { alertTypeRegistryMock } from '../../../../../triggers_actions_ui/public/application/alert_type_registry.mock';
+import { coreMock } from '../../../../../../../src/core/public/mocks';
+import { AlertsContextValue } from '../../../../../triggers_actions_ui/public/application/context/alerts_context';
+// eslint-disable-next-line @kbn/eslint/no-restricted-paths
+import { InventoryMetricConditions } from '../../../../server/lib/alerting/inventory_metric_threshold/types';
+import React from 'react';
+import { Expressions, AlertContextMeta, ExpressionRow } from './expression';
+import { act } from 'react-dom/test-utils';
+// eslint-disable-next-line @kbn/eslint/no-restricted-paths
+import { Comparator } from '../../../../server/lib/alerting/metric_threshold/types';
+import { SnapshotCustomMetricInput } from '../../../../common/http_api/snapshot_api';
+
+jest.mock('../../../containers/source/use_source_via_http', () => ({
+  useSourceViaHttp: () => ({
+    source: { id: 'default' },
+    createDerivedIndexPattern: () => ({ fields: [], title: 'metricbeat-*' }),
+  }),
+}));
+
+const exampleCustomMetric = {
+  id: 'this-is-an-id',
+  field: 'some.system.field',
+  aggregation: 'rate',
+  type: 'custom',
+} as SnapshotCustomMetricInput;
+
+describe('Expression', () => {
+  async function setup(currentOptions: AlertContextMeta) {
+    const alertParams = {
+      criteria: [],
+      nodeType: undefined,
+      filterQueryText: '',
+    };
+
+    const mocks = coreMock.createSetup();
+    const startMocks = coreMock.createStart();
+    const [
+      {
+        application: { capabilities },
+      },
+    ] = await mocks.getStartServices();
+
+    const context: AlertsContextValue<AlertContextMeta> = {
+      http: mocks.http,
+      toastNotifications: mocks.notifications.toasts,
+      actionTypeRegistry: actionTypeRegistryMock.create() as any,
+      alertTypeRegistry: alertTypeRegistryMock.create() as any,
+      docLinks: startMocks.docLinks,
+      capabilities: {
+        ...capabilities,
+        actions: {
+          delete: true,
+          save: true,
+          show: true,
+        },
+      },
+      metadata: currentOptions,
+    };
+
+    const wrapper = mountWithIntl(
+      <Expressions
+        alertsContext={context}
+        alertInterval="1m"
+        alertParams={alertParams as any}
+        errors={[]}
+        setAlertParams={(key, value) => Reflect.set(alertParams, key, value)}
+        setAlertProperty={() => {}}
+      />
+    );
+
+    const update = async () =>
+      await act(async () => {
+        await nextTick();
+        wrapper.update();
+      });
+
+    await update();
+
+    return { wrapper, update, alertParams };
+  }
+
+  it('should prefill the alert using the context metadata', async () => {
+    const currentOptions = {
+      filter: 'foo',
+      nodeType: 'pod',
+      customMetrics: [],
+      options: { metric: { type: 'memory' } },
+    };
+    const { alertParams } = await setup(currentOptions as AlertContextMeta);
+    expect(alertParams.nodeType).toBe('pod');
+    expect(alertParams.filterQueryText).toBe('foo');
+    expect(alertParams.criteria).toEqual([
+      {
+        metric: 'memory',
+        comparator: Comparator.GT,
+        threshold: [],
+        timeSize: 1,
+        timeUnit: 'm',
+      },
+    ]);
+  });
+  describe('using custom metrics', () => {
+    it('should prefill the alert using the context metadata', async () => {
+      const currentOptions = {
+        filter: '',
+        nodeType: 'tx',
+        customMetrics: [exampleCustomMetric],
+        options: { metric: exampleCustomMetric },
+      };
+      const { alertParams, update } = await setup(currentOptions as AlertContextMeta);
+      await update();
+      expect(alertParams.nodeType).toBe('tx');
+      expect(alertParams.filterQueryText).toBe('');
+      expect(alertParams.criteria).toEqual([
+        {
+          metric: 'custom',
+          comparator: Comparator.GT,
+          threshold: [],
+          timeSize: 1,
+          timeUnit: 'm',
+          customMetric: exampleCustomMetric,
+        },
+      ]);
+    });
+  });
+});
+
+describe('ExpressionRow', () => {
+  async function setup(expression: InventoryMetricConditions) {
+    const wrapper = mountWithIntl(
+      <ExpressionRow
+        nodeType="host"
+        canDelete={false}
+        remove={() => {}}
+        addExpression={() => {}}
+        key={1}
+        expressionId={1}
+        setAlertParams={() => {}}
+        errors={{
+          aggField: [],
+          timeSizeUnit: [],
+          timeWindowSize: [],
+          metric: [],
+        }}
+        expression={expression}
+        alertsContextMetadata={{
+          customMetrics: [],
+        }}
+      />
+    );
+
+    const update = async () =>
+      await act(async () => {
+        await nextTick();
+        wrapper.update();
+      });
+
+    await update();
+
+    return { wrapper, update };
+  }
+  const expression = {
+    metric: 'custom',
+    comparator: Comparator.GT,
+    threshold: [],
+    timeSize: 1,
+    timeUnit: 'm',
+    customMetric: exampleCustomMetric,
+  };
+
+  it('loads custom metrics passed in through the expression, even with an empty context', async () => {
+    const { wrapper } = await setup(expression as InventoryMetricConditions);
+    const [valueMatch] =
+      wrapper.html().match('<span class="euiExpression__value">Rate of some.system.field</span>') ??
+      [];
+    expect(valueMatch).toBeTruthy();
+  });
+});
diff --git a/x-pack/plugins/infra/public/alerting/inventory/components/expression.tsx b/x-pack/plugins/infra/public/alerting/inventory/components/expression.tsx
index 7ca17617871ffb..78cabcf3544372 100644
--- a/x-pack/plugins/infra/public/alerting/inventory/components/expression.tsx
+++ b/x-pack/plugins/infra/public/alerting/inventory/components/expression.tsx
@@ -4,7 +4,8 @@
  * you may not use this file except in compliance with the Elastic License.
  */
 
-import { debounce, pick } from 'lodash';
+import { set } from '@elastic/safer-lodash-set';
+import { debounce, pick, uniqBy, isEqual } from 'lodash';
 import { Unit } from '@elastic/datemath';
 import React, { useCallback, useMemo, useEffect, useState, ChangeEvent } from 'react';
 import {
@@ -22,6 +23,7 @@ import {
 } from '@elastic/eui';
 import { FormattedMessage } from '@kbn/i18n/react';
 import { i18n } from '@kbn/i18n';
+import { getCustomMetricLabel } from '../../../../common/formatters/get_custom_metric_label';
 import { toMetricOpt } from '../../../../common/snapshot_metric_i18n';
 import { AlertPreview } from '../../common';
 import { METRIC_INVENTORY_THRESHOLD_ALERT_TYPE_ID } from '../../../../common/alerting/metrics';
@@ -49,22 +51,31 @@ import { hostMetricTypes } from '../../../../common/inventory_models/host/toolba
 import { containerMetricTypes } from '../../../../common/inventory_models/container/toolbar_items';
 import { podMetricTypes } from '../../../../common/inventory_models/pod/toolbar_items';
 import { findInventoryModel } from '../../../../common/inventory_models';
-import { InventoryItemType, SnapshotMetricType } from '../../../../common/inventory_models/types';
+import {
+  InventoryItemType,
+  SnapshotMetricType,
+  SnapshotMetricTypeRT,
+} from '../../../../common/inventory_models/types';
 // eslint-disable-next-line @kbn/eslint/no-restricted-paths
 import { InventoryMetricConditions } from '../../../../server/lib/alerting/inventory_metric_threshold/types';
 import { MetricExpression } from './metric';
 import { NodeTypeExpression } from './node_type';
 import { InfraWaffleMapOptions } from '../../../lib/lib';
 import { convertKueryToElasticSearchQuery } from '../../../utils/kuery';
+import {
+  SnapshotCustomMetricInput,
+  SnapshotCustomMetricInputRT,
+} from '../../../../common/http_api/snapshot_api';
 
 import { validateMetricThreshold } from './validation';
 
 const FILTER_TYPING_DEBOUNCE_MS = 500;
 
-interface AlertContextMeta {
+export interface AlertContextMeta {
   options?: Partial<InfraWaffleMapOptions>;
   nodeType?: InventoryItemType;
   filter?: string;
+  customMetrics?: SnapshotCustomMetricInput[];
 }
 
 interface Props {
@@ -89,6 +100,7 @@ const defaultExpression = {
   threshold: [],
   timeSize: 1,
   timeUnit: 'm',
+  customMetric: undefined,
 } as InventoryMetricConditions;
 
 export const Expressions: React.FC<Props> = (props) => {
@@ -204,6 +216,9 @@ export const Expressions: React.FC<Props> = (props) => {
         {
           ...defaultExpression,
           metric: md.options.metric!.type,
+          customMetric: SnapshotCustomMetricInputRT.is(md.options.metric)
+            ? md.options.metric
+            : undefined,
         } as InventoryMetricConditions,
       ]);
     } else {
@@ -282,6 +297,7 @@ export const Expressions: React.FC<Props> = (props) => {
               setAlertParams={updateParams}
               errors={errors[idx] || emptyError}
               expression={e || {}}
+              alertsContextMetadata={alertsContext.metadata}
             />
           );
         })}
@@ -389,6 +405,7 @@ interface ExpressionRowProps {
   addExpression(): void;
   remove(id: number): void;
   setAlertParams(id: number, params: Partial<InventoryMetricConditions>): void;
+  alertsContextMetadata: AlertsContextValue<AlertContextMeta>['metadata'];
 }
 
 const StyledExpressionRow = euiStyled(EuiFlexGroup)`
@@ -402,14 +419,48 @@ const StyledExpression = euiStyled.div`
 `;
 
 export const ExpressionRow: React.FC<ExpressionRowProps> = (props) => {
-  const { setAlertParams, expression, errors, expressionId, remove, canDelete } = props;
-  const { metric, comparator = Comparator.GT, threshold = [] } = expression;
+  const {
+    setAlertParams,
+    expression,
+    errors,
+    expressionId,
+    remove,
+    canDelete,
+    alertsContextMetadata,
+  } = props;
+  const { metric, comparator = Comparator.GT, threshold = [], customMetric } = expression;
+  const [customMetrics, updateCustomMetrics] = useState<SnapshotCustomMetricInput[]>([]);
+
+  // Create and uniquify a list of custom metrics including:
+  // - The alert metadata context (which only gives us custom metrics on the inventory page)
+  // - The custom metric stored in the expression (necessary when editing this alert without having
+  //    access to the metadata context)
+  // - Whatever custom metrics were previously stored in this list (to preserve the custom metric in the dropdown
+  //    if the user edits the alert and switches away from the custom metric)
+  useEffect(() => {
+    const ctxCustomMetrics = alertsContextMetadata?.customMetrics ?? [];
+    const expressionCustomMetrics = customMetric ? [customMetric] : [];
+    const newCustomMetrics = uniqBy(
+      [...customMetrics, ...ctxCustomMetrics, ...expressionCustomMetrics],
+      (cm: SnapshotCustomMetricInput) => cm.id
+    );
+    if (!isEqual(customMetrics, newCustomMetrics)) updateCustomMetrics(newCustomMetrics);
+  }, [alertsContextMetadata, customMetric, customMetrics, updateCustomMetrics]);
 
   const updateMetric = useCallback(
-    (m?: SnapshotMetricType) => {
-      setAlertParams(expressionId, { ...expression, metric: m });
+    (m?: SnapshotMetricType | string) => {
+      const newMetric = SnapshotMetricTypeRT.is(m) ? m : 'custom';
+      const newAlertParams = { ...expression, metric: newMetric };
+      if (newMetric === 'custom' && customMetrics) {
+        set(
+          newAlertParams,
+          'customMetric',
+          customMetrics.find((cm) => cm.id === m)
+        );
+      }
+      setAlertParams(expressionId, newAlertParams);
     },
-    [expressionId, expression, setAlertParams]
+    [expressionId, expression, setAlertParams, customMetrics]
   );
 
   const updateComparator = useCallback(
@@ -446,6 +497,7 @@ export const ExpressionRow: React.FC<ExpressionRowProps> = (props) => {
         break;
       case 'host':
         myMetrics = hostMetricTypes;
+
         break;
       case 'pod':
         myMetrics = podMetricTypes;
@@ -454,8 +506,17 @@ export const ExpressionRow: React.FC<ExpressionRowProps> = (props) => {
         myMetrics = containerMetricTypes;
         break;
     }
-    return myMetrics.map(toMetricOpt);
-  }, [props.nodeType]);
+    const baseMetricOpts = myMetrics.map(toMetricOpt);
+    const customMetricOpts = customMetrics
+      ? customMetrics.map((m, i) => ({
+          text: getCustomMetricLabel(m),
+          value: m.id,
+        }))
+      : [];
+    return [...baseMetricOpts, ...customMetricOpts];
+  }, [props.nodeType, customMetrics]);
+
+  const selectedMetricValue = metric === 'custom' && customMetric ? customMetric.id : metric!;
 
   return (
     <>
@@ -465,8 +526,8 @@ export const ExpressionRow: React.FC<ExpressionRowProps> = (props) => {
             <StyledExpression>
               <MetricExpression
                 metric={{
-                  value: metric!,
-                  text: ofFields.find((v) => v?.value === metric)?.text || '',
+                  value: selectedMetricValue,
+                  text: ofFields.find((v) => v?.value === selectedMetricValue)?.text || '',
                 }}
                 metrics={
                   ofFields.filter((m) => m !== undefined && m.value !== undefined) as Array<{
@@ -568,4 +629,5 @@ const metricUnit: Record<string, { label: string }> = {
   s3DownloadBytes: { label: 'bytes' },
   sqsOldestMessage: { label: 'seconds' },
   rdsLatency: { label: 'ms' },
+  custom: { label: '' },
 };
diff --git a/x-pack/plugins/infra/public/alerting/inventory/components/metric.tsx b/x-pack/plugins/infra/public/alerting/inventory/components/metric.tsx
index ff859a95a3d9da..2e5ccbe1a42768 100644
--- a/x-pack/plugins/infra/public/alerting/inventory/components/metric.tsx
+++ b/x-pack/plugins/infra/public/alerting/inventory/components/metric.tsx
@@ -18,13 +18,12 @@ import {
 import { EuiPopoverTitle, EuiButtonIcon } from '@elastic/eui';
 // eslint-disable-next-line @kbn/eslint/no-restricted-paths
 import { IErrorObject } from '../../../../../triggers_actions_ui/public/types';
-import { SnapshotMetricType } from '../../../../common/inventory_models/types';
 
 interface Props {
-  metric?: { value: SnapshotMetricType; text: string };
+  metric?: { value: string; text: string };
   metrics: Array<{ value: string; text: string }>;
   errors: IErrorObject;
-  onChange: (metric?: SnapshotMetricType) => void;
+  onChange: (metric?: string) => void;
   popupPosition?:
     | 'upCenter'
     | 'upLeft'
@@ -104,7 +103,7 @@ export const MetricExpression = ({ metric, metrics, errors, onChange, popupPosit
                 renderOption={(o: any) => o.label}
                 onChange={(selectedOptions) => {
                   if (selectedOptions.length > 0) {
-                    onChange(selectedOptions[0].value as SnapshotMetricType);
+                    onChange(selectedOptions[0].value);
                     setAggFieldPopoverOpen(false);
                   } else {
                     onChange();
diff --git a/x-pack/plugins/infra/public/alerting/inventory/hooks/use_inventory_alert_prefill.ts b/x-pack/plugins/infra/public/alerting/inventory/hooks/use_inventory_alert_prefill.ts
index d659057b95ed92..a57f9cafa5e192 100644
--- a/x-pack/plugins/infra/public/alerting/inventory/hooks/use_inventory_alert_prefill.ts
+++ b/x-pack/plugins/infra/public/alerting/inventory/hooks/use_inventory_alert_prefill.ts
@@ -5,20 +5,26 @@
  */
 
 import { useState } from 'react';
-import { SnapshotMetricInput } from '../../../../common/http_api/snapshot_api';
+import {
+  SnapshotMetricInput,
+  SnapshotCustomMetricInput,
+} from '../../../../common/http_api/snapshot_api';
 import { InventoryItemType } from '../../../../common/inventory_models/types';
 
 export const useInventoryAlertPrefill = () => {
   const [nodeType, setNodeType] = useState<InventoryItemType>('host');
   const [filterQuery, setFilterQuery] = useState<string | undefined>();
   const [metric, setMetric] = useState<SnapshotMetricInput>({ type: 'cpu' });
+  const [customMetrics, setCustomMetrics] = useState<SnapshotCustomMetricInput[]>([]);
 
   return {
     nodeType,
     filterQuery,
     metric,
+    customMetrics,
     setNodeType,
     setFilterQuery,
     setMetric,
+    setCustomMetrics,
   };
 };
diff --git a/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/index.tsx b/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/index.tsx
index aae787c8c03959..96169810e02a84 100644
--- a/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/index.tsx
+++ b/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/index.tsx
@@ -8,13 +8,13 @@ import { EuiPopover } from '@elastic/eui';
 import { i18n } from '@kbn/i18n';
 import React, { useState, useCallback } from 'react';
 import { IFieldType } from 'src/plugins/data/public';
+import { getCustomMetricLabel } from '../../../../../../../common/formatters/get_custom_metric_label';
 import {
   SnapshotMetricInput,
   SnapshotCustomMetricInput,
   SnapshotCustomMetricInputRT,
 } from '../../../../../../../common/http_api/snapshot_api';
 import { CustomMetricForm } from './custom_metric_form';
-import { getCustomMetricLabel } from './get_custom_metric_label';
 import { MetricsContextMenu } from './metrics_context_menu';
 import { ModeSwitcher } from './mode_switcher';
 import { MetricsEditMode } from './metrics_edit_mode';
diff --git a/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/metrics_context_menu.tsx b/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/metrics_context_menu.tsx
index 7ab90297ebbb3a..71a000d3165b60 100644
--- a/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/metrics_context_menu.tsx
+++ b/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/metrics_context_menu.tsx
@@ -5,6 +5,7 @@
  */
 import React, { useCallback } from 'react';
 import { EuiContextMenuPanelDescriptor, EuiContextMenu } from '@elastic/eui';
+import { getCustomMetricLabel } from '../../../../../../../common/formatters/get_custom_metric_label';
 import {
   SnapshotMetricInput,
   SnapshotCustomMetricInput,
@@ -14,7 +15,6 @@ import {
   SnapshotMetricTypeRT,
   SnapshotMetricType,
 } from '../../../../../../../common/inventory_models/types';
-import { getCustomMetricLabel } from './get_custom_metric_label';
 
 interface Props {
   options: Array<{ text: string; value: string }>;
diff --git a/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/metrics_edit_mode.tsx b/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/metrics_edit_mode.tsx
index 649dcc4282d670..e75885ccbc9173 100644
--- a/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/metrics_edit_mode.tsx
+++ b/x-pack/plugins/infra/public/pages/metrics/inventory_view/components/waffle/metric_control/metrics_edit_mode.tsx
@@ -6,8 +6,8 @@
 import React from 'react';
 import { EuiFlexItem, EuiFlexGroup, EuiButtonIcon } from '@elastic/eui';
 import { i18n } from '@kbn/i18n';
+import { getCustomMetricLabel } from '../../../../../../../common/formatters/get_custom_metric_label';
 import { SnapshotCustomMetricInput } from '../../../../../../../common/http_api/snapshot_api';
-import { getCustomMetricLabel } from './get_custom_metric_label';
 import {
   EuiTheme,
   withTheme,
diff --git a/x-pack/plugins/infra/public/pages/metrics/inventory_view/hooks/use_waffle_options.test.ts b/x-pack/plugins/infra/public/pages/metrics/inventory_view/hooks/use_waffle_options.test.ts
index 579073e9500d0c..d44d8fca4faba3 100644
--- a/x-pack/plugins/infra/public/pages/metrics/inventory_view/hooks/use_waffle_options.test.ts
+++ b/x-pack/plugins/infra/public/pages/metrics/inventory_view/hooks/use_waffle_options.test.ts
@@ -20,6 +20,7 @@ jest.mock('react-router-dom', () => ({
 // reassign them, so we can't make these both part of the same object
 let PREFILL_NODETYPE: WaffleOptionsState['nodeType'] | undefined;
 let PREFILL_METRIC: WaffleOptionsState['metric'] | undefined;
+let PREFILL_CUSTOM_METRICS: WaffleOptionsState['customMetrics'] | undefined;
 jest.mock('../../../../alerting/use_alert_prefill', () => ({
   useAlertPrefillContext: () => ({
     inventoryPrefill: {
@@ -29,6 +30,9 @@ jest.mock('../../../../alerting/use_alert_prefill', () => ({
       setMetric(metric: WaffleOptionsState['metric']) {
         PREFILL_METRIC = metric;
       },
+      setCustomMetrics(customMetrics: WaffleOptionsState['customMetrics']) {
+        PREFILL_CUSTOM_METRICS = customMetrics;
+      },
     },
   }),
 }));
@@ -39,6 +43,7 @@ describe('useWaffleOptions', () => {
   beforeEach(() => {
     PREFILL_NODETYPE = undefined;
     PREFILL_METRIC = undefined;
+    PREFILL_CUSTOM_METRICS = undefined;
   });
 
   it('should sync the options to the inventory alert preview context', () => {
@@ -47,6 +52,15 @@ describe('useWaffleOptions', () => {
     const newOptions = {
       nodeType: 'pod',
       metric: { type: 'memory' },
+      customMetrics: [
+        {
+          type: 'custom',
+          id:
+            "i don't want to bother to copy and paste an actual uuid so instead i'm going to smash my keyboard skjdghsjodkyjheurvjnsgn",
+          aggregation: 'avg',
+          field: 'hey.system.are.you.good',
+        },
+      ],
     } as WaffleOptionsState;
     act(() => {
       result.current.changeNodeType(newOptions.nodeType);
@@ -58,5 +72,10 @@ describe('useWaffleOptions', () => {
     });
     rerender();
     expect(PREFILL_METRIC).toEqual(newOptions.metric);
+    act(() => {
+      result.current.changeCustomMetrics(newOptions.customMetrics);
+    });
+    rerender();
+    expect(PREFILL_CUSTOM_METRICS).toEqual(newOptions.customMetrics);
   });
 });
diff --git a/x-pack/plugins/infra/public/pages/metrics/inventory_view/hooks/use_waffle_options.ts b/x-pack/plugins/infra/public/pages/metrics/inventory_view/hooks/use_waffle_options.ts
index 8059d1ad12a3a1..35d069adc939ec 100644
--- a/x-pack/plugins/infra/public/pages/metrics/inventory_view/hooks/use_waffle_options.ts
+++ b/x-pack/plugins/infra/public/pages/metrics/inventory_view/hooks/use_waffle_options.ts
@@ -125,9 +125,10 @@ export const useWaffleOptions = () => {
 
   const { inventoryPrefill } = useAlertPrefillContext();
   useEffect(() => {
-    const { setNodeType, setMetric } = inventoryPrefill;
+    const { setNodeType, setMetric, setCustomMetrics } = inventoryPrefill;
     setNodeType(state.nodeType);
     setMetric(state.metric);
+    setCustomMetrics(state.customMetrics);
   }, [state, inventoryPrefill]);
 
   return {
diff --git a/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/evaluate_condition.ts b/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/evaluate_condition.ts
index 3b795810b39f09..9be6a4b52157c9 100644
--- a/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/evaluate_condition.ts
+++ b/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/evaluate_condition.ts
@@ -5,6 +5,7 @@
  */
 import { mapValues, last, first } from 'lodash';
 import moment from 'moment';
+import { SnapshotCustomMetricInput } from '../../../../common/http_api/snapshot_api';
 import {
   isTooManyBucketsPreviewException,
   TOO_MANY_BUCKETS_PREVIEW_EXCEPTION,
@@ -37,7 +38,7 @@ export const evaluateCondition = async (
   filterQuery?: string,
   lookbackSize?: number
 ): Promise<Record<string, ConditionResult>> => {
-  const { comparator, metric } = condition;
+  const { comparator, metric, customMetric } = condition;
   let { threshold } = condition;
 
   const timerange = {
@@ -55,7 +56,8 @@ export const evaluateCondition = async (
     metric,
     timerange,
     sourceConfiguration,
-    filterQuery
+    filterQuery,
+    customMetric
   );
 
   threshold = threshold.map((n) => convertMetricValue(metric, n));
@@ -93,19 +95,24 @@ const getData = async (
   metric: SnapshotMetricType,
   timerange: InfraTimerangeInput,
   sourceConfiguration: InfraSourceConfiguration,
-  filterQuery?: string
+  filterQuery?: string,
+  customMetric?: SnapshotCustomMetricInput
 ) => {
   const snapshot = new InfraSnapshot();
   const esClient = <Hit = {}, Aggregation = undefined>(
     options: CallWithRequestParams
   ): Promise<InfraDatabaseSearchResponse<Hit, Aggregation>> => callCluster('search', options);
 
+  const metrics = [
+    metric === 'custom' ? (customMetric as SnapshotCustomMetricInput) : { type: metric },
+  ];
+
   const options = {
     filterQuery: parseFilterQuery(filterQuery),
     nodeType,
     groupBy: [],
     sourceConfiguration,
-    metrics: [{ type: metric }],
+    metrics,
     timerange,
     includeTimeseries: Boolean(timerange.lookbackSize),
   };
diff --git a/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/inventory_metric_threshold_executor.ts b/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/inventory_metric_threshold_executor.ts
index 7b816f2f225b53..db1ff26ee1810d 100644
--- a/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/inventory_metric_threshold_executor.ts
+++ b/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/inventory_metric_threshold_executor.ts
@@ -6,6 +6,7 @@
 import { first, get, last } from 'lodash';
 import { i18n } from '@kbn/i18n';
 import moment from 'moment';
+import { getCustomMetricLabel } from '../../../../common/formatters/get_custom_metric_label';
 import { toMetricOpt } from '../../../../common/snapshot_metric_i18n';
 import { AlertStates, InventoryMetricConditions } from './types';
 import { AlertExecutorOptions } from '../../../../../alerts/server';
@@ -77,27 +78,19 @@ export const createInventoryMetricThresholdExecutor = (libs: InfraBackendLibs) =
     let reason;
     if (nextState === AlertStates.ALERT) {
       reason = results
-        .map((result) => {
-          if (!result[item]) return '';
-          const resultWithVerboseMetricName = {
-            ...result[item],
-            metric: toMetricOpt(result[item].metric)?.text || result[item].metric,
-            currentValue: formatMetric(result[item].metric, result[item].currentValue),
-          };
-          return buildFiredAlertReason(resultWithVerboseMetricName);
-        })
+        .map((result) => buildReasonWithVerboseMetricName(result[item], buildFiredAlertReason))
         .join('\n');
     }
     if (alertOnNoData) {
       if (nextState === AlertStates.NO_DATA) {
         reason = results
           .filter((result) => result[item].isNoData)
-          .map((result) => buildNoDataAlertReason(result[item]))
+          .map((result) => buildReasonWithVerboseMetricName(result[item], buildNoDataAlertReason))
           .join('\n');
       } else if (nextState === AlertStates.ERROR) {
         reason = results
           .filter((result) => result[item].isError)
-          .map((result) => buildErrorAlertReason(result[item].metric))
+          .map((result) => buildReasonWithVerboseMetricName(result[item], buildErrorAlertReason))
           .join('\n');
       }
     }
@@ -121,6 +114,20 @@ export const createInventoryMetricThresholdExecutor = (libs: InfraBackendLibs) =
   }
 };
 
+const buildReasonWithVerboseMetricName = (resultItem: any, buildReason: (r: any) => string) => {
+  if (!resultItem) return '';
+  const resultWithVerboseMetricName = {
+    ...resultItem,
+    metric:
+      toMetricOpt(resultItem.metric)?.text ||
+      (resultItem.metric === 'custom'
+        ? getCustomMetricLabel(resultItem.customMetric)
+        : resultItem.metric),
+    currentValue: formatMetric(resultItem.metric, resultItem.currentValue),
+  };
+  return buildReason(resultWithVerboseMetricName);
+};
+
 const mapToConditionsLookup = (
   list: any[],
   mapFn: (value: any, index: number, array: any[]) => unknown
@@ -140,10 +147,6 @@ export const FIRED_ACTIONS = {
 };
 
 const formatMetric = (metric: SnapshotMetricType, value: number) => {
-  // if (SnapshotCustomMetricInputRT.is(metric)) {
-  //   const formatter = createFormatterForMetric(metric);
-  //   return formatter(val);
-  // }
   const metricFormatter = get(METRIC_FORMATTERS, metric, METRIC_FORMATTERS.count);
   if (value == null) {
     return '';
diff --git a/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/register_inventory_metric_threshold_alert_type.ts b/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/register_inventory_metric_threshold_alert_type.ts
index f664a59acd165e..14d1acf0e4a9f6 100644
--- a/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/register_inventory_metric_threshold_alert_type.ts
+++ b/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/register_inventory_metric_threshold_alert_type.ts
@@ -27,6 +27,15 @@ const condition = schema.object({
   timeUnit: schema.string(),
   timeSize: schema.number(),
   metric: schema.string(),
+  customMetric: schema.maybe(
+    schema.object({
+      type: schema.literal('custom'),
+      id: schema.string(),
+      field: schema.string(),
+      aggregation: schema.string(),
+      label: schema.maybe(schema.string()),
+    })
+  ),
 });
 
 export const registerMetricInventoryThresholdAlertType = (libs: InfraBackendLibs) => ({
diff --git a/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/types.ts b/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/types.ts
index 86c77e6d7459ac..06f5efaf9eb369 100644
--- a/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/types.ts
+++ b/x-pack/plugins/infra/server/lib/alerting/inventory_metric_threshold/types.ts
@@ -4,6 +4,7 @@
  * you may not use this file except in compliance with the Elastic License.
  */
 import { Unit } from '@elastic/datemath';
+import { SnapshotCustomMetricInput } from '../../../../common/http_api/snapshot_api';
 import { SnapshotMetricType } from '../../../../common/inventory_models/types';
 import { Comparator, AlertStates } from '../common/types';
 
@@ -18,4 +19,5 @@ export interface InventoryMetricConditions {
   sourceId?: string;
   threshold: number[];
   comparator: Comparator;
+  customMetric?: SnapshotCustomMetricInput;
 }