Create metric events for service desk emails

When a new service desk email is sent, the relevant counter is updated.
These events will be used for monitoring service desk functionality.

Changelog: added

app/mailers/emails/service_desk.rb

@@ -20,7 +20,9 @@ module Emails
       options = service_desk_options(email_sender, 'thank_you', @issue.external_author)
                   .merge(subject: "Re: #{subject_base}")
 
-      mail_new_thread(@issue, options)
+      mail_new_thread(@issue, options).tap do
+        Gitlab::Metrics::BackgroundTransaction.current&.add_event(:service_desk_thank_you_email)
+      end
     end
 
     def service_desk_new_note_email(issue_id, note_id, recipient)
@@ -31,7 +33,9 @@ module Emails
       options = service_desk_options(email_sender, 'new_note', recipient)
                   .merge(subject: subject_base)
 
-      mail_answer_thread(@issue, options)
+      mail_answer_thread(@issue, options).tap do
+        Gitlab::Metrics::BackgroundTransaction.current&.add_event(:service_desk_new_note_email)
+      end
     end
 
     private

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -130,6 +130,8 @@ The following metrics are available:
 | `gitlab_ci_pipeline_security_orchestration_policy_processing_duration_seconds` | Histogram   | 13.12    | Time in seconds it takes to process Security Policies in CI/CD pipeline                                                                    |                                                           |
 | `gitlab_ci_difference_live_vs_actual_minutes`                    | Histogram   | 13.12    | Difference between CI minute consumption counted while jobs were running (live) vs when jobs are complete (actual). Used to enforce CI minute consumption limits on long running jobs. | `plan` |
 | `gitlab_spamcheck_request_duration_seconds`                      | Histogram   | 13.12   | The duration for requests between Rails and the anti-spam engine                                                        |                                                           |
+| `service_desk_thank_you_email`                                   | Counter     | 14.0    | Total number of email responses to new service desk emails                                                            |                                                           |
+| `service_desk_new_note_email`                                    | Counter     | 14.0    | Total number of email notifications on new service desk comment                                                       |                                                           |
 
 ## Metrics controlled by a feature flag
 

spec/mailers/emails/service_desk_spec.rb

@@ -115,6 +115,16 @@ RSpec.describe Emails::ServiceDesk do
     end
   end
 
+  shared_examples 'notification with metric event' do |event_type|
+    it 'adds metric event' do
+      metric_transaction = double('Gitlab::Metrics::WebTransaction', increment: true, observe: true)
+      allow(::Gitlab::Metrics::BackgroundTransaction).to receive(:current).and_return(metric_transaction)
+      expect(metric_transaction).to receive(:add_event).with(event_type)
+
+      subject.content_type
+    end
+  end
+
   describe '.service_desk_thank_you_email' do
     let_it_be(:reply_in_subject) { true }
     let_it_be(:default_text) do
@@ -124,6 +134,7 @@ RSpec.describe Emails::ServiceDesk do
     subject { ServiceEmailClass.service_desk_thank_you_email(issue.id) }
 
     it_behaves_like 'read template from repository', 'thank_you'
+    it_behaves_like 'notification with metric event', :service_desk_thank_you_email
 
     context 'handling template markdown' do
       context 'with a simple text' do
@@ -164,6 +175,7 @@ RSpec.describe Emails::ServiceDesk do
     subject { ServiceEmailClass.service_desk_new_note_email(issue.id, note.id, email) }
 
     it_behaves_like 'read template from repository', 'new_note'
+    it_behaves_like 'notification with metric event', :service_desk_new_note_email
 
     context 'handling template markdown' do
       context 'with a simple text' do