[Metricbeat] Fix "ID" event generator of Google Cloud module (#17608)

Error is unrelated, merging

CHANGELOG.next.asciidoc

@@ -150,6 +150,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fix Unix socket path in memcached. {pull}17512[17512]
 - Fix vsphere VM dashboard host aggregation visualizations. {pull}17555[17555]
 - Metricbeat no longer needs to be started strictly after Logstash for `logstash-xpack` module to report correct data. {issue}17261[17261] {pull}17497[17497]
+- Fix "ID" event generator of Google Cloud module {issue}17160[17160] {pull}17608[17608]
 
 *Packetbeat*
 

x-pack/metricbeat/module/googlecloud/stackdriver/timeseries.go

@@ -17,28 +17,29 @@ import (
 func (m *MetricSet) timeSeriesGrouped(ctx context.Context, gcpService googlecloud.MetadataService, tss []*monitoringpb.TimeSeries, e *incomingFieldExtractor) (map[string][]KeyValuePoint, error) {
 	eventGroups := make(map[string][]KeyValuePoint)
 
-	for _, ts := range tss {
-		if gcpService == nil {
-			gcpService = googlecloud.NewStackdriverMetadataServiceForTimeSeries(ts)
-		}
+	metadataService := gcpService
 
+	for _, ts := range tss {
 		keyValues, err := e.extractTimeSeriesMetricValues(ts)
 		if err != nil {
 			return nil, err
 		}
 
 		sdCollectorInputData := googlecloud.NewStackdriverCollectorInputData(ts, m.config.ProjectID, m.config.Zone, m.config.Region)
+		if gcpService == nil {
+			metadataService = googlecloud.NewStackdriverMetadataServiceForTimeSeries(ts)
+		}
 
 		for i := range keyValues {
 			sdCollectorInputData.Timestamp = &keyValues[i].Timestamp
 
-			id, err := gcpService.ID(ctx, sdCollectorInputData)
+			id, err := metadataService.ID(ctx, sdCollectorInputData)
 			if err != nil {
 				m.Logger().Errorf("error trying to retrieve ID from metric event '%v'", err)
 				continue
 			}
 
-			metadataCollectorData, err := gcpService.Metadata(ctx, sdCollectorInputData.TimeSeries)
+			metadataCollectorData, err := metadataService.Metadata(ctx, sdCollectorInputData.TimeSeries)
 			if err != nil {
 				m.Logger().Error("error trying to retrieve labels from metric event")
 				continue