[v10.0.x] CloudMonitoring: Improve parsing of GCM labels (#69812)

CloudMonitoring: Improve parsing of GCM labels (#69800)

Update parsing of GCM labels

- Include parsing of metadata labels

(cherry picked from commit 9fc1de62d5)

Co-authored-by: Andreas Christou <andreas.christou@grafana.com>

pkg/tsdb/cloudmonitoring/time_series_query_test.go

@@ -110,6 +110,7 @@ func TestTimeSeriesQuery(t *testing.T) {
 		labels, ok := custom["labels"].(gdata.Labels)
 		require.True(t, ok)
 		assert.Equal(t, "6724404429462225363", labels["resource.label.instance_id"])
+		assert.Equal(t, "test-app", labels["metadata.label.app"])
 	})
 
 	t.Run("includes time interval", func(t *testing.T) {