Adds Service Desk custom email enabled to Service Ping

Adds the counts.service_desk_custom_email_enabled
metric to Service Ping. Provides the number of projects
that have Service Desk custom email enabled.

config/metrics/counts_all/20231204142552_service_desk_custom_email_enabled.yml

+---
+key_path: counts.service_desk_custom_email_enabled
+description: "Number of projects that have Service Desk custom email enabled"
+product_section: seg
+product_stage: service management
+product_group: respond
+value_type: number
+status: active
+milestone: "16.7"
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/138662
+time_frame: all
+data_source: database
+data_category: optional
+instrumentation_class: CountServiceDeskCustomEmailEnabledMetric
+distribution:
+- ce
+- ee
+tier:
+- free
+- premium
+- ultimate

db/migrate/20231204144300_add_index_service_desk_settings_on_custom_email_enabled.rb

+# frozen_string_literal: true
+
+class AddIndexServiceDeskSettingsOnCustomEmailEnabled < Gitlab::Database::Migration[2.2]
+  milestone '16.7'
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'index_service_desk_settings_on_custom_email_enabled'
+
+  def up
+    add_concurrent_index :service_desk_settings, :custom_email_enabled, name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :service_desk_settings, INDEX_NAME
+  end
+end

db/schema_migrations/20231204144300

+385471e69c2099130d0758d2fe6ac27b9211204759de04f3fc0a79b84b5a1633
\ No newline at end of file

db/structure.sql

@@ -34514,6 +34514,8 @@ CREATE INDEX index_service_desk_custom_email_verifications_on_triggerer_id ON se
 CREATE INDEX index_service_desk_enabled_projects_on_id_creator_id_created_at ON projects USING btree (id, creator_id, created_at) WHERE (service_desk_enabled = true);
+CREATE INDEX index_service_desk_settings_on_custom_email_enabled ON service_desk_settings USING btree (custom_email_enabled);
+
 CREATE INDEX index_service_desk_settings_on_file_template_project_id ON service_desk_settings USING btree (file_template_project_id);
 CREATE UNIQUE INDEX index_shards_on_name ON shards USING btree (name);

lib/gitlab/usage/metrics/instrumentations/count_service_desk_custom_email_enabled_metric.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Usage
+    module Metrics
+      module Instrumentations
+        class CountServiceDeskCustomEmailEnabledMetric < DatabaseMetric
+          operation :count
+
+          relation do
+            ServiceDeskSetting.where(custom_email_enabled: true)
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/usage/metrics/instrumentations/count_service_desk_custom_email_enabled_metric_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Usage::Metrics::Instrumentations::CountServiceDeskCustomEmailEnabledMetric, feature_category: :service_ping do
+  let_it_be(:project) { create(:project) }
+  let_it_be(:credential) { create(:service_desk_custom_email_credential, project: project) }
+  let_it_be(:verification) { create(:service_desk_custom_email_verification, :finished, project: project) }
+  let_it_be(:setting) do
+    create(:service_desk_setting, project: project, custom_email: 'support@example.com', custom_email_enabled: true)
+  end
+
+  let(:expected_value) { 1 }
+
+  it_behaves_like 'a correct instrumented metric value', { time_frame: 'all', data_source: 'database' }
+end