[Metricbeat] gcp: log metrics without descriptors (#26982)

Metrics listed in the light-weight modules are checked with GCP
Stackdriver APIs to gather further information about them.

In case the metric is not valid, the request to the API returns an empty
response and the metric in question is excluded from collection.
There is no error returned from APIs.

In such a case a log line at error level is produced, notifying the user
of the wrong behaviour.

(cherry picked from commit 452f0fa)

x-pack/metricbeat/module/gcp/metrics/metricset.go

@@ -245,7 +245,8 @@ func (m *MetricSet) metricDescriptor(ctx context.Context, client *monitoring.Met
 
 	for _, sdc := range m.MetricsConfig {
 		for _, mt := range sdc.MetricTypes {
-			req.Filter = fmt.Sprintf(`metric.type = starts_with("%s")`, sdc.AddPrefixTo(mt))
+			id := sdc.AddPrefixTo(mt)
+			req.Filter = fmt.Sprintf(`metric.type = starts_with("%s")`, id)
 			it := client.ListMetricDescriptors(ctx, req)
 
 			for {
@@ -263,6 +264,14 @@ func (m *MetricSet) metricDescriptor(ctx context.Context, client *monitoring.Met
 				if err == iterator.Done {
 					break
 				}
+
+			}
+
+			// NOTE: if a metric is not added to the metricsWithMeta map is not collected subsequently.
+			// Such a case is an error, as the configuration is explicitly requesting a metric that the beat
+			// is not able to collect, so we provide a logging statement for this behaviour.
+			if _, ok := metricsWithMeta[id]; !ok {
+				m.Logger().Errorf("%s metric descriptor is empty, this metric will not be collected", mt)
 			}
 		}
 	}