Loki: Add backend functionality to parse structured metadata from Loki (#77361)

* prepare backend for structured metadata

* add `lokiStructuredMetadata` feature toggle

* use `lokiStructuredMetadata` feature flag

* add field type check to `labelTypesField`

* remove fixme

* fix feature toggle

* add field in dataplane mode

* use `data.Labels` where possible

* adjust framing tests

* improve verbiage

* improve naming

* update tests to "attributes"

pkg/services/featuremgmt/registry.go

@@ -893,6 +893,13 @@ var (
 			Owner:           grafanaAlertingSquad,
 			RequiresRestart: true,
 		},
+		{
+			Name:         "lokiStructuredMetadata",
+			Description:  "Enables the loki data source to request structured metadata from the Loki server",
+			Stage:        FeatureStageExperimental,
+			FrontendOnly: false,
+			Owner:        grafanaObservabilityLogsSquad,
+		},
 		{
 			Name:         "teamHttpHeaders",
 			Description:  "Enables datasources to apply team headers to the client requests",

pkg/services/featuremgmt/toggles_gen.csv

@@ -119,6 +119,7 @@ kubernetesPlaylists,experimental,@grafana/grafana-app-platform-squad,false,false
 cloudWatchBatchQueries,preview,@grafana/aws-datasources,false,false,false,false
 navAdminSubsections,experimental,@grafana/grafana-frontend-platform,false,false,false,false
 recoveryThreshold,experimental,@grafana/alerting-squad,false,false,true,false
+lokiStructuredMetadata,experimental,@grafana/observability-logs,false,false,false,false
 teamHttpHeaders,experimental,@grafana/identity-access-team,false,false,false,false
 awsDatasourcesNewFormStyling,experimental,@grafana/aws-datasources,false,false,false,true
 cachingOptimizeSerializationMemoryUsage,experimental,@grafana/grafana-operator-experience-squad,false,false,false,false

pkg/services/featuremgmt/toggles_gen.go

@@ -487,6 +487,10 @@ const (
 	// Enables feature recovery threshold (aka hysteresis) for threshold server-side expression
 	FlagRecoveryThreshold = "recoveryThreshold"
 
+	// FlagLokiStructuredMetadata
+	// Enables the loki data source to request structured metadata from the Loki server
+	FlagLokiStructuredMetadata = "lokiStructuredMetadata"
+
 	// FlagTeamHttpHeaders
 	// Enables datasources to apply team headers to the client requests
 	FlagTeamHttpHeaders = "teamHttpHeaders"