[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/einterfaces/metrics.go

@@ -95,4 +95,10 @@ type MetricsInterface interface {
 
 	SetReplicaLagAbsolute(node string, value float64)
 	SetReplicaLagTime(node string, value float64)
+
+	IncrementNotificationCounter(notificationType model.NotificationType)
+	IncrementNotificationAckCounter(notificationType model.NotificationType)
+	IncrementNotificationSuccessCounter(notificationType model.NotificationType)
+	IncrementNotificationErrorCounter(notificationType model.NotificationType, errorReason model.NotificationReason)
+	IncrementNotificationNotSentCounter(notificationType model.NotificationType, notSentReason model.NotificationReason)
 }