Metrics for shared channels (#26199)

* add metrics definitions for shared channels

server/platform/services/sharedchannel/sync_send.go

@@ -178,12 +178,25 @@ func (scs *Service) doSync() time.Duration {
 	var task syncTask
 	var ok bool
 	var shortestWait time.Duration
+	metrics := scs.server.GetMetrics()
+
+	if metrics != nil {
+		scs.mux.Lock()
+		size := len(scs.tasks)
+		scs.mux.Unlock()
+		metrics.ObserveSharedChannelsQueueSize(int64(size))
+	}
 
 	for {
 		task, ok, shortestWait = scs.removeOldestTask()
 		if !ok {
 			break
 		}
+
+		if metrics != nil {
+			metrics.ObserveSharedChannelsTaskInQueueDuration(time.Since(task.AddedAt).Seconds())
+		}
+
 		if err := scs.processTask(task); err != nil {
 			// put task back into map so it will update again
 			if task.incRetry() {
@@ -236,18 +249,22 @@ func (scs *Service) removeOldestTask() (syncTask, bool, time.Duration) {
 
 // processTask updates one or more remote clusters with any new channel content.
 func (scs *Service) processTask(task syncTask) error {
-	var err error
-	var remotes []*model.RemoteCluster
+	// map is used to ensure remotes don't get sync'd twice, such as when
+	// they have the autoinvited flag and have explicitly subscribed to a channel.
+	remotesMap := make(map[string]*model.RemoteCluster)
 
 	if task.remoteID == "" {
 		filter := model.RemoteClusterQueryFilter{
 			InChannel:     task.channelID,
 			OnlyConfirmed: true,
 		}
-		remotes, err = scs.server.GetStore().RemoteCluster().GetAll(filter)
+		remotes, err := scs.server.GetStore().RemoteCluster().GetAll(filter)
 		if err != nil {
 			return err
 		}
+		for _, r := range remotes {
+			remotesMap[r.RemoteId] = r
+		}
 
 		// add all remotes that have the autoinvited option.
 		filter = model.RemoteClusterQueryFilter{
@@ -257,7 +274,9 @@ func (scs *Service) processTask(task syncTask) error {
 		if err != nil {
 			return err
 		}
-		remotes = append(remotes, remotesAutoInvited...)
+		for _, r := range remotesAutoInvited {
+			remotesMap[r.RemoteId] = r
+		}
 	} else {
 		rc, err := scs.server.GetStore().RemoteCluster().Get(task.remoteID)
 		if err != nil {
@@ -266,10 +285,10 @@ func (scs *Service) processTask(task syncTask) error {
 		if !rc.IsOnline() {
 			return fmt.Errorf("Failed updating shared channel '%s' for offline remote cluster '%s'", task.channelID, rc.DisplayName)
 		}
-		remotes = []*model.RemoteCluster{rc}
+		remotesMap[rc.RemoteId] = rc
 	}
 
-	for _, rc := range remotes {
+	for _, rc := range remotesMap {
 		rtask := task
 		rtask.remoteID = rc.RemoteId
 		if err := scs.syncForRemote(rtask, rc); err != nil {