Implement action mailer delivery metrics observer

config/initializers/action_mailer_hooks.rb

@@ -3,5 +3,10 @@ unless Gitlab.config.gitlab.email_enabled
   ActionMailer::Base.logger = nil
 end
 
-ActionMailer::Base.register_interceptor(::Gitlab::Email::Hook::AdditionalHeadersInterceptor)
-ActionMailer::Base.register_interceptor(::Gitlab::Email::Hook::EmailTemplateInterceptor)
+ActionMailer::Base.register_interceptors(
+  ::Gitlab::Email::Hook::AdditionalHeadersInterceptor,
+  ::Gitlab::Email::Hook::EmailTemplateInterceptor,
+  ::Gitlab::Email::Hook::DeliveryMetricsObserver
+)
+
+ActionMailer::Base.register_observer(::Gitlab::Email::Hook::DeliveryMetricsObserver)

lib/gitlab/email/hook/delivery_metrics_observer.rb

+module Gitlab
+  module Email
+    module Hook
+      class DeliveryMetricsObserver
+        extend Gitlab::Utils::StrongMemoize
+
+        def self.delivering_email(_message)
+          delivery_attempts_counter.increment
+        end
+
+        def self.delivered_email(_message)
+          delivered_emails_counter.increment
+        end
+
+        def self.delivery_attempts_counter
+          strong_memoize(:delivery_attempts_counter) do
+            Gitlab::Metrics.counter(:gitlab_emails_delivery_attempts_total,
+                                    'Counter of total emails delivery attempts')
+          end
+        end
+
+        def self.delivered_emails_counter
+          strong_memoize(:delivered_emails_counter) do
+            Gitlab::Metrics.counter(:gitlab_emails_delivered_total,
+                                    'Counter of total emails delievered')
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/email/hook/delivery_metrics_observer_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Email::Hook::DeliveryMetricsObserver do
+  let(:email) do
+    ActionMailer::Base.mail(to: 'test@example.com',
+                            from: 'info@example.com',
+                            body: 'hello')
+  end
+
+  context 'when email has been delivered' do
+    it 'increments both email delivery metrics' do
+      expect(described_class.delivery_attempts_counter).to receive(:increment)
+      expect(described_class.delivered_emails_counter).to receive(:increment)
+
+      email.deliver_now
+    end
+  end
+
+  context 'when email has not been delivered due to an error' do
+    before do
+      allow(email.delivery_method).to receive(:deliver!)
+        .and_raise(StandardError, 'Some SMTP error')
+    end
+
+    it 'increments only delivery attempt metric' do
+      expect(described_class.delivery_attempts_counter)
+        .to receive(:increment)
+      expect(described_class.delivered_emails_counter)
+        .not_to receive(:increment)
+
+      expect { email.deliver_now }
+        .to raise_error(StandardError, 'Some SMTP error')
+    end
+  end
+end