changefeedccl: give notice when metrics_label set without child_metrics

Resolves #75682

Surfaces a notice of
```
server.child_metrics.enabled is set to false, metrics will only be published to
the '<scope>' label when it is set to true"
```
When child_metrics setting isn't enabled during changefeed creation

Release note (enterprise change): Changefeeds created/altered with a
metrics_label set while server.child_metrics.enabled is false will now provide
the user a notice upon creation.

<what was there before: Previously, ...>
<why it needed to change: This was inadequate because ...>
<what you did about it: To address this, this patch ...>

pkg/ccl/changefeedccl/BUILD.bazel

@@ -74,6 +74,7 @@ go_library(
         "//pkg/scheduledjobs",
         "//pkg/scheduledjobs/schedulebase",
         "//pkg/security/username",
+        "//pkg/server/status",
         "//pkg/server/telemetry",
         "//pkg/settings",
         "//pkg/settings/cluster",

pkg/ccl/changefeedccl/changefeed_stmt.go

@@ -30,6 +30,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver"
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/protectedts"
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/protectedts/ptpb"
+	"github.com/cockroachdb/cockroach/pkg/server/status"
 	"github.com/cockroachdb/cockroach/pkg/server/telemetry"
 	"github.com/cockroachdb/cockroach/pkg/settings"
 	"github.com/cockroachdb/cockroach/pkg/settings/cluster"
@@ -581,6 +582,14 @@ func createChangefeedJobRecord(
 					"Otherwise, please re-run with a different %[1]q value.",
 				changefeedbase.OptMetricsScope, defaultSLIScope)
 		}
+
+		if !status.ChildMetricsEnabled.Get(&p.ExecCfg().Settings.SV) {
+			p.BufferClientNotice(ctx, pgnotice.Newf(
+				"%s is set to false, metrics will only be published to the '%s' label when it is set to true",
+				status.ChildMetricsEnabled.Key(),
+				scope,
+			))
+		}
 	}
 
 	details.Opts = opts.AsMap()

pkg/ccl/changefeedccl/changefeed_test.go

@@ -7658,6 +7658,31 @@ func TestPartitionSpans(t *testing.T) {
 	}
 }
 
+func TestChangefeedMetricsScopeNotice(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+
+	s, stopServer := makeServer(t)
+	defer stopServer()
+	sqlDB := sqlutils.MakeSQLRunner(s.DB)
+	sqlDB.Exec(t, "CREATE table foo (i int)")
+	sqlDB.Exec(t, `SET CLUSTER SETTING server.child_metrics.enabled = false`)
+
+	sqlCreate := "CREATE CHANGEFEED FOR d.foo INTO 'null://' WITH metrics_label='scope'"
+	expectNotice(t, s.Server, sqlCreate, `server.child_metrics.enabled is set to false, metrics will only be published to the 'scope' label when it is set to true`)
+
+	var jobID string
+	sqlDB.QueryRow(t, `SELECT job_id FROM [SHOW JOBS] where job_type='CHANGEFEED'`).Scan(&jobID)
+	sqlDB.Exec(t, "PAUSE JOB $1", jobID)
+	sqlDB.CheckQueryResultsRetry(
+		t,
+		fmt.Sprintf(`SELECT count(*) FROM [SHOW JOBS] WHERE job_type='CHANGEFEED' AND status='%s'`, jobs.StatusPaused),
+		[][]string{{"1"}},
+	)
+
+	sqlAlter := fmt.Sprintf("ALTER CHANGEFEED %s SET metrics_label='other'", jobID)
+	expectNotice(t, s.Server, sqlAlter, `server.child_metrics.enabled is set to false, metrics will only be published to the 'other' label when it is set to true`)
+}
+
 // TestPubsubValidationErrors tests error messages during pubsub sink URI validations.
 func TestPubsubValidationErrors(t *testing.T) {
 	defer leaktest.AfterTest(t)()

pkg/server/status/recorder.go

@@ -86,7 +86,8 @@ type storeMetrics interface {
 	Registry() *metric.Registry
 }
 
-var childMetricsEnabled = settings.RegisterBoolSetting(
+// ChildMetricsEnabled enables exporting of additional prometheus time series with extra labels
+var ChildMetricsEnabled = settings.RegisterBoolSetting(
 	settings.TenantWritable, "server.child_metrics.enabled",
 	"enables the exporting of child metrics, additional prometheus time series with extra labels",
 	false).WithPublic()
@@ -245,7 +246,7 @@ func (mr *MetricsRecorder) ScrapeIntoPrometheus(pm *metric.PrometheusExporter) {
 			log.Warning(context.TODO(), "MetricsRecorder asked to scrape metrics before NodeID allocation")
 		}
 	}
-	includeChildMetrics := childMetricsEnabled.Get(&mr.settings.SV)
+	includeChildMetrics := ChildMetricsEnabled.Get(&mr.settings.SV)
 	pm.ScrapeRegistry(mr.mu.nodeRegistry, includeChildMetrics)
 	for _, reg := range mr.mu.storeRegistries {
 		pm.ScrapeRegistry(reg, includeChildMetrics)