[MM-57067][MM-57006][MM-57328] Acknowledge websocket POSTED events that may result in a notification on the web client (#26604)

* Acknowledge POSTED events that may result in a notification

* Remove temporary metrics

* Add Desktop App support

* Merge'd

* Add some tests

* PR feedback

* Rework window is focused check

* Oops

* Move mentions/followers ACK to posted ACK broadcast hook

---------

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

server/channels/wsapi/system.go

@@ -5,10 +5,12 @@ package wsapi
 
 import (
 	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/shared/mlog"
 )
 
 func (api *API) InitSystem() {
 	api.Router.Handle("ping", api.APIWebSocketHandler(ping))
+	api.Router.Handle(model.WebsocketPostedNotifyAck, api.APIWebSocketHandler(api.websocketNotificationAck))
 }
 
 func ping(req *model.WebSocketRequest) (map[string]any, *model.AppError) {
@@ -20,3 +22,18 @@ func ping(req *model.WebSocketRequest) (map[string]any, *model.AppError) {
 
 	return data, nil
 }
+
+func (api *API) websocketNotificationAck(req *model.WebSocketRequest) (map[string]any, *model.AppError) {
+	// Log the ACKs if necessary
+	api.App.NotificationsLog().Debug("Websocket notification acknowledgment",
+		mlog.String("type", model.TypeWebsocket),
+		mlog.String("user_id", req.Session.UserId),
+		mlog.Any("user_agent", req.Data["user_agent"]),
+		mlog.Any("post_id", req.Data["post_id"]),
+		mlog.Any("result", req.Data["result"]),
+		mlog.Any("reason", req.Data["reason"]),
+		mlog.Any("data", req.Data["data"]),
+	)
+
+	return nil, nil
+}