controller: clearer metrics handler shutdown logs

The metrics handler previously always emitted an error when being shut
down, even if the error was nil. This was okay before because we never
actually properly shutdown the metrics handler before, but now that
we're going to, this needs to be clearer.

This makes the metrics handler only emit an error when there is an
error, and emit an info message when it successfully shuts down.

pkg/controller/common/metrics.go

@@ -62,7 +62,12 @@ func StartMetricsListener(addr string, stopCh <-chan struct{}) {
 		}
 	}()
 	<-stopCh
-	if err := s.Shutdown(context.Background()); err != http.ErrServerClosed {
-		glog.Errorf("error stopping metrics listener: %v", err)
+	if err := s.Shutdown(context.Background()); err != nil {
+		if err != http.ErrServerClosed {
+			glog.Errorf("error stopping metrics listener: %v", err)
+		}
+	} else {
+		glog.Infof("Metrics listener successfully stopped")
 	}
+
 }