Use a unique metric tag for each email handler

Otherwise there is a possibility that within the same
Sidekiq process, that same event with different
metrics_params will cause Prometheus to
throw an error, "labels must have the same signature"

ee/changelogs/unreleased/bw-fix-servicedesk-events.yml

+---
+title: Fix service desk emails not creating issues intermittently
+merge_request: 16577
+author:
+type: fixed

ee/lib/gitlab/email/handler/ee/service_desk_handler.rb

@@ -39,6 +39,10 @@ module Gitlab
             super.merge(project: project&.full_path)
           end
 
+          def metrics_event
+            :receive_email_service_desk
+          end
+
           private
 
           attr_reader :project_id, :project_path

lib/gitlab/email/handler/base_handler.rb

@@ -24,6 +24,14 @@ module Gitlab
         def metrics_params
           { handler: self.class.name }
         end
+
+        # Each handler should use it's own metric event.  Otherwise there
+        # is a possibility that within the same Sidekiq process, that same
+        # event with different metrics_params will cause Prometheus to
+        # throw an error
+        def metrics_event
+          raise NotImplementedError
+        end
       end
     end
   end

lib/gitlab/email/handler/create_issue_handler.rb

@@ -48,6 +48,10 @@ module Gitlab
         end
         # rubocop: enable CodeReuse/ActiveRecord
 
+        def metrics_event
+          :receive_email_create_issue
+        end
+
         private
 
         def create_issue

lib/gitlab/email/handler/create_merge_request_handler.rb

@@ -54,6 +54,10 @@ module Gitlab
           super.merge(includes_patches: patch_attachments.any?)
         end
 
+        def metrics_event
+          :receive_email_create_merge_request
+        end
+
         private
 
         def build_merge_request

lib/gitlab/email/handler/create_note_handler.rb

@@ -32,6 +32,10 @@ module Gitlab
             record_name: 'comment')
         end
 
+        def metrics_event
+          :receive_email_create_note
+        end
+
         private
 
         def author

lib/gitlab/email/handler/unsubscribe_handler.rb

@@ -36,6 +36,10 @@ module Gitlab
           noteable.unsubscribe(sent_notification.recipient)
         end
 
+        def metrics_event
+          :receive_email_unsubscribe
+        end
+
         private
 
         attr_reader :reply_token

lib/gitlab/email/receiver.rb

@@ -39,9 +39,9 @@ module Gitlab
 
         raise UnknownIncomingEmail unless handler
 
-        Gitlab::Metrics.add_event(:receive_email, handler.metrics_params)
-
-        handler.execute
+        handler.execute.tap do
+          Gitlab::Metrics.add_event(handler.metrics_event, handler.metrics_params)
+        end
       end
 
       private

spec/lib/gitlab/email/receiver_spec.rb

@@ -14,6 +14,7 @@ describe Gitlab::Email::Receiver do
 
       allow(handler).to receive(:execute)
       allow(handler).to receive(:metrics_params)
+      allow(handler).to receive(:metrics_event)
     end
 
     it "finds the mail key" do
@@ -46,4 +47,12 @@ describe Gitlab::Email::Receiver do
       expect { receiver.execute }.to raise_error(Gitlab::Email::AutoGeneratedEmailError)
     end
   end
+
+  it "requires all handlers to have a unique metric_event" do
+    events = Gitlab::Email::Handler.handlers.map do |handler|
+      handler.new(Mail::Message.new, 'gitlabhq/gitlabhq+auth_token').metrics_event
+    end
+
+    expect(events.uniq.count).to eq events.count
+  end
 end