Send metrics for notifications delivered by API

This metric is the complement of the same metrics sent from the
notifications controller.
The metrics sent by the notifications model correspond to the API
requests, whereas the metrics sent by the controller correspond to the
UI requests (bulk update).

src/api/app/models/notification.rb

@@ -54,9 +54,11 @@ def track_notification_creation
                             "notification.create,notifiable_type=#{notifiable_type},web=#{web},rss=#{rss} value=1")
   end
 
+  # This is only called when the request come from the API. The UI performs 'update_all' that does not trigger callbacks.
+  # This metrics complements the same metrics tracked in Webui::Users::NotificationsController#send_notifications_information_rabbitmq
   def track_notification_delivered
     RabbitmqBus.send_to_bus('metrics',
-                            "notification.delivered,notifiable_type=#{notifiable_type},web=#{web},rss=#{rss} value=1")
+                            "notification,action=#{delivered ? 'read' : 'unread'} value=1")
   end
 end