Unregister prom gauges when recycling cluster watcher

Fixes #11839

When in `restartClusterWatcher` we fail to connect to the target cluster
for whatever reason, the function gets called again 10s later, and tries
to register the same prometheus metrics without unregistering them
first, which generates warnings.

The problem lies in `NewRemoteClusterServiceWatcher`, which instantiates
the remote kube-api client and registers the metrics, returning a nil
object if the client can't connect. `cleanupWorkers` at the beginning of
`restartClusterWatcher` won't unregister those metrics because of that
nil object.

This fix reorders `NewRemoteClusterServiceWatcher` so that an object is
returned even when there's an error, so cleanup on that object can be
performed.

multicluster/service-mirror/cluster_watcher.go

@@ -173,10 +173,6 @@ func NewRemoteClusterServiceWatcher(
 	if err != nil {
 		return nil, fmt.Errorf("cannot initialize api for target cluster %s: %w", clusterName, err)
 	}
-	_, err = remoteAPI.Client.Discovery().ServerVersion()
-	if err != nil {
-		return nil, fmt.Errorf("cannot connect to api for target cluster %s: %w", clusterName, err)
-	}
 
 	// Create k8s event recorder
 	eventBroadcaster := record.NewBroadcaster()
@@ -188,7 +184,7 @@ func NewRemoteClusterServiceWatcher(
 	})
 
 	stopper := make(chan struct{})
-	return &RemoteClusterServiceWatcher{
+	rcsw := RemoteClusterServiceWatcher{
 		serviceMirrorNamespace: serviceMirrorNamespace,
 		link:                   link,
 		remoteAPIClient:        remoteAPI,
@@ -207,7 +203,14 @@ func NewRemoteClusterServiceWatcher(
 		headlessServicesEnabled: enableHeadlessSvc,
 		// always instantiate the gatewayAlive=true to prevent unexpected service fail fast
 		gatewayAlive: true,
-	}, nil
+	}
+
+	_, err = remoteAPI.Client.Discovery().ServerVersion()
+	if err != nil {
+		return &rcsw, fmt.Errorf("cannot connect to api for target cluster %s: %w", clusterName, err)
+	}
+
+	return &rcsw, nil
 }
 
 func (rcsw *RemoteClusterServiceWatcher) mirroredResourceName(remoteName string) string {