[MM-59361] custom groups & read only channels (#28892)

* added group mention, read only view and post event tracking

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/channel.go

@@ -1585,7 +1585,7 @@ func (a *App) addUserToChannel(c request.CTX, user *model.User, channel *model.C
 		a.Srv().telemetryService.SendTelemetryForFeature(
 			telemetry.TrackGuestFeature,
 			"add_guest_to_channel",
-			map[string]any{"user_actual_id": user.Id})
+			map[string]any{telemetry.TrackPropertyUser: user.Id})
 	}
 
 	a.Srv().Platform().InvalidateChannelCacheForUser(user.Id)
@@ -2663,10 +2663,12 @@ func (a *App) SetActiveChannel(c request.CTX, userID string, channelID string) *
 
 	oldStatus := model.StatusOffline
 
+	oldChannelID := ""
 	if err != nil {
 		status = &model.Status{UserId: userID, Status: model.StatusOnline, Manual: false, LastActivityAt: model.GetMillis(), ActiveChannel: channelID}
 	} else {
 		oldStatus = status.Status
+		oldChannelID = status.ActiveChannel
 		status.ActiveChannel = channelID
 		if !status.Manual && channelID != "" {
 			status.Status = model.StatusOnline
@@ -2680,6 +2682,24 @@ func (a *App) SetActiveChannel(c request.CTX, userID string, channelID string) *
 		a.Srv().Platform().BroadcastStatus(status)
 	}
 
+	if channelID != "" && oldChannelID != channelID {
+		// is this a read-only channel?
+		isReadOnly, ircErr := a.Srv().Store().Channel().IsReadOnlyChannel(channelID)
+		if ircErr != nil {
+			mlog.Warn("Error trying to check if it is a readonly channel", mlog.Err(ircErr))
+		}
+		if isReadOnly {
+			a.Srv().telemetryService.SendTelemetryForFeature(
+				telemetry.TrackReadOnlyFeature,
+				"read_only_channel_viewed",
+				map[string]any{
+					telemetry.TrackPropertyUser:    userID,
+					telemetry.TrackPropertyChannel: channelID,
+				},
+			)
+		}
+	}
+
 	return nil
 }