feat: metrics endpoint (#332)

Co-authored-by: David May <1301201+wass3r@users.noreply.github.com>

content/reference/api/metrics/_index.md

@@ -0,0 +1,8 @@
+---
+title: "Metrics"
+linkTitle: "Metrics"
+description: >
+  Learn how to use API endpoints for retrieving platform metrics.
+---
+
+The below endpoints are available to retrieve metrics for a running Vela installation.

content/reference/api/metrics/metrics.md

@@ -0,0 +1,67 @@
+---
+title: "Metrics"
+linkTitle: "Metrics"
+description: >
+  Learn how to list metrics.
+---
+
+## Endpoint
+
+```
+GET  /metrics
+```
+
+## Query Parameters
+
+The following parameters are used to configure the endpoint:
+
+| Query Parameter    | Description               |
+| ------- | ------------------------- |
+|	`user_count` | count of total users |
+|	`repo_count` | count of total repos |
+|	`build_count` | count of total builds |
+|	`running_build_count` | count of builds with status 'running' |
+|	`failure_build_count` | count of builds with status 'failure' |
+|	`pending_build_count` | count of builds with status 'pending' |
+|	`killed_build_count` | count of builds with status 'killed' |
+|	`success_build_count` | count of builds with status 'success' |
+|	`error_build_count` | count of builds with status 'error' |
+|	`step_image_count` | count of total steps executed with each unique image |
+|	`step_status_count` | count of total steps with each status |
+|	`service_image_count` | count of total services executed with each unique image |
+|	`service_status_count` | count of total services with each status |
+|	`worker_build_limit` | count of worker build limits |
+|	`active_worker_count` | count of active workers |
+|	`inactive_worker_count` | count of inactive workers |
+
+
+## Permissions
+
+No permission restrictions for this endpoint.
+
+## Responses
+
+| Status Code | Description                                             |
+| ----------- | ------------------------------------------------------- |
+| `200`       | indicates the request has succeeded                     |
+| `500`       | indicates a server failure while processing the request |
+
+## Sample
+
+#### Request
+
+```sh
+curl \
+  -X GET \
+  "http://127.0.0.1:8080/metrics?pending_build_count=true"
+```
+
+#### Response
+
+```json
+# HELP go_gc_duration_seconds A summary of the pause duration of garbage collection cycles.
+# TYPE go_gc_duration_seconds summary
+go_gc_duration_seconds{quantile="0"} 4.0986e-05
+# TYPE vela_totals gauge
+vela_totals{field="status",resource="build",value="pending"} 31
+```