Add metrics events for incoming emails

This lets us track how many incoming emails a GitLab instance is processing, by
email type (handler) and by project (where applicable).

changelogs/unreleased/add-email-receiver-metrics.yml

+---
+title: Add metrics events for incoming emails
+merge_request:
+author:

lib/gitlab/email/receiver.rb

@@ -32,6 +32,10 @@ module Gitlab
 
         raise UnknownIncomingEmail unless handler
 
+        Gitlab::Metrics.add_event(:receive_email,
+                                  project: handler.try(:project),
+                                  handler: handler.class.name)
+
         handler.execute
       end