Add telemetry for projects inheriting instance settings

This commit adds new counters following the pattern:
projects_inheriting_instance_#{service_name}_active

The new counters allow us to know the actual usage to determine how it's
getting used, instead of just that it's getting used.

changelogs/unreleased/204802-telemetry-projects-inheriting-instance-settings.yml

+---
+title: Add telemetry for projects inheriting instance settings
+merge_request: 38561
+author:
+type: other

lib/gitlab/usage_data.rb

@@ -358,6 +358,7 @@ module Gitlab
           response["projects_#{service_name}_active".to_sym] = count(Service.active.where(template: false, instance: false, type: "#{service_name}_service".camelize))
           response["templates_#{service_name}_active".to_sym] = count(Service.active.where(template: true, type: "#{service_name}_service".camelize))
           response["instances_#{service_name}_active".to_sym] = count(Service.active.where(instance: true, type: "#{service_name}_service".camelize))
+          response["projects_inheriting_instance_#{service_name}_active".to_sym] = count(Service.active.where.not(inherit_from_id: nil).where(type: "#{service_name}_service".camelize))
         end.merge(jira_usage, jira_import_usage)
         # rubocop: enable UsageData/LargeTable:
       end

spec/factories/usage_data.rb

@@ -24,7 +24,8 @@ FactoryBot.define do
       create(:service, project: projects[2], type: 'SlackService', active: true)
       create(:service, project: projects[2], type: 'MattermostService', active: false)
       create(:service, :template, type: 'MattermostService', active: true)
-      create(:service, :instance, type: 'MattermostService', active: true)
+      matermost_instance = create(:service, :instance, type: 'MattermostService', active: true)
+      create(:service, project: projects[1], type: 'MattermostService', active: true, inherit_from_id: matermost_instance.id)
       create(:service, project: projects[2], type: 'CustomIssueTrackerService', active: true)
       create(:project_error_tracking_setting, project: projects[0])
       create(:project_error_tracking_setting, project: projects[1], enabled: false)

spec/lib/gitlab/usage_data_spec.rb

@@ -343,9 +343,10 @@ RSpec.describe Gitlab::UsageData, :aggregate_failures do
       expect(count_data[:projects_slack_active]).to eq(2)
       expect(count_data[:projects_slack_slash_commands_active]).to eq(1)
       expect(count_data[:projects_custom_issue_tracker_active]).to eq(1)
-      expect(count_data[:projects_mattermost_active]).to eq(0)
+      expect(count_data[:projects_mattermost_active]).to eq(1)
       expect(count_data[:templates_mattermost_active]).to eq(1)
       expect(count_data[:instances_mattermost_active]).to eq(1)
+      expect(count_data[:projects_inheriting_instance_mattermost_active]).to eq(1)
       expect(count_data[:projects_with_repositories_enabled]).to eq(3)
       expect(count_data[:projects_with_error_tracking_enabled]).to eq(1)
       expect(count_data[:projects_with_alerts_service_enabled]).to eq(1)