[MM-57066][MM-57329] Added metrics for all notification stopping points, consolidated categories between metrics and logging (#26799)

* [MM-57066] Add metric counters for notification events

* Some small changes

* Account for Metrics() sometimes being nil

* Fix test (again)

* Fix more tests

* A few changes from testing - added success counter

* Missed a mock

* Lint

* Add feature flag for notification monitoring

server/channels/api4/system.go

@@ -641,13 +641,16 @@ func pushNotificationAck(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	c.App.CountNotificationAck(model.NotificationTypePush)
+
 	err := c.App.SendAckToPushProxy(&ack)
 	if ack.IsIdLoaded {
 		if err != nil {
+			c.App.CountNotificationReason(model.NotificationStatusError, model.NotificationTypePush, model.NotificationReasonPushProxySendError)
 			c.App.NotificationsLog().Error("Notification ack not sent to push proxy",
-				mlog.String("type", model.TypePush),
-				mlog.String("status", model.StatusServerError),
-				mlog.String("reason", model.ReasonServerError),
+				mlog.String("type", model.NotificationTypePush),
+				mlog.String("status", model.NotificationStatusError),
+				mlog.String("reason", model.NotificationReasonPushProxySendError),
 				mlog.String("ack_id", ack.Id),
 				mlog.String("push_type", ack.NotificationType),
 				mlog.String("post_id", ack.PostId),
@@ -656,6 +659,8 @@ func pushNotificationAck(c *Context, w http.ResponseWriter, r *http.Request) {
 				mlog.Int("received_at", ack.ClientReceivedAt),
 				mlog.Err(err),
 			)
+		} else {
+			c.App.CountNotificationReason(model.NotificationStatusSuccess, model.NotificationTypePush, model.NotificationReason(""))
 		}
 		// Return post data only when PostId is passed.
 		if ack.PostId != "" && ack.NotificationType == model.PushTypeMessage {
@@ -687,6 +692,7 @@ func pushNotificationAck(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	c.App.CountNotificationReason(model.NotificationStatusSuccess, model.NotificationTypePush, model.NotificationReason(""))
 	ReturnStatusOK(w)
 }