Use background transaction for events

Email receiver is used only by sidekiq so it should use
BackgroundTransaction.

lib/gitlab/email/receiver.rb

@@ -20,7 +20,7 @@ module Gitlab
         raise UnknownIncomingEmail unless handler
 
         handler.execute.tap do
-          Gitlab::Metrics.add_event(handler.metrics_event, handler.metrics_params)
+          Gitlab::Metrics::BackgroundTransaction.current&.add_event(handler.metrics_event, handler.metrics_params)
         end
       end
 

spec/lib/gitlab/email/receiver_spec.rb

@@ -5,9 +5,13 @@ require 'spec_helper'
 RSpec.describe Gitlab::Email::Receiver do
   include_context :email_shared_context
 
-  shared_examples 'correctly finds the mail key' do
-    specify do
+  shared_examples 'correctly finds the mail key and adds metric event' do
+    let(:metric_transaction) { double('Gitlab::Metrics::WebTransaction') }
+
+    specify :aggregate_failures do
       expect(Gitlab::Email::Handler).to receive(:for).with(an_instance_of(Mail::Message), 'gitlabhq/gitlabhq+auth_token').and_return(handler)
+      expect(::Gitlab::Metrics::BackgroundTransaction).to receive(:current).and_return(metric_transaction)
+      expect(metric_transaction).to receive(:add_event).with(handler.metrics_event, handler.metrics_params)
 
       receiver.execute
     end
@@ -30,7 +34,7 @@ RSpec.describe Gitlab::Email::Receiver do
     context 'when in a Delivered-To header' do
       let(:email_raw) { fixture_file('emails/forwarded_new_issue.eml') }
 
-      it_behaves_like 'correctly finds the mail key'
+      it_behaves_like 'correctly finds the mail key and adds metric event'
 
       it 'parses the metadata' do
         expect(metadata[:delivered_to]). to eq(["incoming+gitlabhq/gitlabhq+auth_token@appmail.example.com", "support@example.com"])
@@ -40,7 +44,7 @@ RSpec.describe Gitlab::Email::Receiver do
     context 'when in an Envelope-To header' do
       let(:email_raw) { fixture_file('emails/envelope_to_header.eml') }
 
-      it_behaves_like 'correctly finds the mail key'
+      it_behaves_like 'correctly finds the mail key and adds metric event'
 
       it 'parses the metadata' do
         expect(metadata[:envelope_to]). to eq(["incoming+gitlabhq/gitlabhq+auth_token@appmail.example.com"])
@@ -50,7 +54,7 @@ RSpec.describe Gitlab::Email::Receiver do
     context 'when in an X-Envelope-To header' do
       let(:email_raw) { fixture_file('emails/x_envelope_to_header.eml') }
 
-      it_behaves_like 'correctly finds the mail key'
+      it_behaves_like 'correctly finds the mail key and adds metric event'
 
       it 'parses the metadata' do
         expect(metadata[:x_envelope_to]). to eq(["incoming+gitlabhq/gitlabhq+auth_token@appmail.example.com"])
@@ -60,7 +64,7 @@ RSpec.describe Gitlab::Email::Receiver do
     context 'when enclosed with angle brackets in an Envelope-To header' do
       let(:email_raw) { fixture_file('emails/envelope_to_header_with_angle_brackets.eml') }
 
-      it_behaves_like 'correctly finds the mail key'
+      it_behaves_like 'correctly finds the mail key and adds metric event'
     end
   end