Add Prometheus active projects to usage data

Collect the number of projects with active Prometheus monitoring.

lib/gitlab/usage_data.rb

@@ -40,6 +40,7 @@ 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,
             remote_mirrors: RemoteMirror.count,

spec/lib/gitlab/usage_data_spec.rb

@@ -54,6 +54,7 @@ describe Gitlab::UsageData do
         milestones
         notes
         projects
+        projects_prometheus_active
         pages_domains
         protected_branches
         releases