PMM-8489 fix: topology in mongos (#322)

* fix: topology in mongos

* Changelog 322

Author: relzhong

CHANGELOG

@@ -1,5 +1,6 @@
 Release 0.20.6 (unreleased)
 
+Fixed bug PMM-8489: Failed to get topology labels when the target server is mongos
 Improvement \#283 : Add storage stats to collection metrics (Thanks Masaru Hoshi @qlik-trial)
 
 

exporter/topology_info.go

@@ -115,11 +115,11 @@ func (t *topologyInfo) loadLabels(ctx context.Context) error {
 	}
 	t.labels[labelClusterID] = cid
 
+	// Standalone instances or mongos instances won't have a replicaset state
 	state, err := util.MyState(ctx, t.client)
-	if err != nil {
-		return errors.Wrapf(ErrCannotGetTopologyLabels, "error getting replicaset state: %s", err)
+	if err == nil {
+		t.labels[labelReplicasetState] = fmt.Sprintf("%d", state)
 	}
-	t.labels[labelReplicasetState] = fmt.Sprintf("%d", state)
 
 	return nil
 }