[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/public/model/feature_flags.go

@@ -55,6 +55,8 @@ type FeatureFlags struct {
 	ChannelBookmarks bool
 
 	WebSocketEventScope bool
+
+	NotificationMonitoring bool
 }
 
 func (f *FeatureFlags) SetDefaults() {
@@ -77,6 +79,7 @@ func (f *FeatureFlags) SetDefaults() {
 	f.CloudDedicatedExportUI = false
 	f.ChannelBookmarks = false
 	f.WebSocketEventScope = false
+	f.NotificationMonitoring = true
 }
 
 // ToMap returns the feature flags as a map[string]string