[receiver/prometheus] speed up tests by setting skipOffsetting (#32341)

Fixes #32298

This PR modifies prometheusreceiver to allow to set the skipOffsetting
option on its scrape option config. This option is private, so it can
only be set by reflection.

This removes the random offset to start added to scraping prometheus
metrics, so we may get faster CI builds.

---------

Co-authored-by: Alex Boten <223565+codeboten@users.noreply.github.com>
Co-authored-by: David Ashpole <dashpole@google.com>
Co-authored-by: Evan Bradley <11745660+evan-bradley@users.noreply.github.com>

receiver/prometheusreceiver/metrics_receiver.go

@@ -12,9 +12,11 @@ import (
 	"net/http"
 	"net/url"
 	"os"
+	"reflect"
 	"regexp"
 	"sync"
 	"time"
+	"unsafe"
 
 	"github.com/go-kit/log"
 	"github.com/mitchellh/hashstructure/v2"
@@ -54,6 +56,7 @@ type pReceiver struct {
 	httpClient        *http.Client
 	registerer        prometheus.Registerer
 	unregisterMetrics func()
+	skipOffsetting    bool // for testing only
 }
 
 // New creates a new prometheus.Receiver reference.
@@ -299,13 +302,24 @@ func (r *pReceiver) initPrometheusComponents(ctx context.Context, logger log.Log
 		return err
 	}
 
-	scrapeManager, err := scrape.NewManager(&scrape.Options{
+	opts := &scrape.Options{
 		PassMetadataInContext: true,
 		ExtraMetrics:          r.cfg.ReportExtraScrapeMetrics,
 		HTTPClientOptions: []commonconfig.HTTPClientOption{
 			commonconfig.WithUserAgent(r.settings.BuildInfo.Command + "/" + r.settings.BuildInfo.Version),
 		},
-	}, logger, store, r.registerer)
+	}
+
+	// for testing only
+	if r.skipOffsetting {
+		optsValue := reflect.ValueOf(opts).Elem()
+		field := optsValue.FieldByName("skipOffsetting")
+		reflect.NewAt(field.Type(), unsafe.Pointer(field.UnsafeAddr())).
+			Elem().
+			Set(reflect.ValueOf(true))
+	}
+
+	scrapeManager, err := scrape.NewManager(opts, logger, store, r.registerer)
 	if err != nil {
 		return err
 	}

receiver/prometheusreceiver/metrics_receiver_helper_test.go

@@ -660,6 +660,7 @@ func testComponent(t *testing.T, targets []*testData, alterConfig func(*Config),
 
 	cms := new(consumertest.MetricsSink)
 	receiver := newPrometheusReceiver(receivertest.NewNopCreateSettings(), config, cms)
+	receiver.skipOffsetting = true
 
 	require.NoError(t, receiver.Start(ctx, componenttest.NewNopHost()))
 	// verify state after shutdown is called