vmon: add missing key labels for gce instance metrics.

Change-Id: Ica6974504c031381986fa7f0a74793737a552d47
diff --git a/vmon/gceinstance.go b/vmon/gceinstance.go
index 7c1e0e5..cf22fc6 100644
--- a/vmon/gceinstance.go
+++ b/vmon/gceinstance.go
@@ -457,7 +457,7 @@
 	if err != nil {
 		return err
 	}
-	if err := sendDataToGCM(s, md, value, timeStr, "", "", metricName); err != nil {
+	if err := sendDataToGCM(s, md, value, timeStr, "", "", instance.name, instance.zone, metricName); err != nil {
 		return fmt.Errorf("failed to write timeseries: %v", err)
 	}
 	return nil