Add prometheus services to usage ping

doc/user/admin_area/settings/usage_statistics.md

@@ -62,6 +62,7 @@ The total number of the following is sent back to GitLab Inc.:
 - Deploy keys
 - Pages
 - Project Services
+- Projects using the Prometheus service
 - Issue Boards
 - CI Runners
 - Deployments

lib/gitlab/usage_data.rb

@@ -36,6 +36,7 @@ module Gitlab
             notes: Note.count,
             pages_domains: PagesDomain.count,
             projects: Project.count,
+            projects_prometheus_active: PrometheusService.active.count,
             protected_branches: ProtectedBranch.count,
             releases: Release.count,
             services: Service.where(active: true).count,

spec/lib/gitlab/usage_data_spec.rb

@@ -44,6 +44,7 @@ describe Gitlab::UsageData do
         milestones
         notes
         projects
+        projects_prometheus_active
         pages_domains
         protected_branches
         releases