Instrument the HousekeepingService class

This allows us to track how much time is spent in updating the
"pushes_since_gc" column as well as the time needed to obtain the lease.

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.7.0 (unreleased)
+  - The Projects::HousekeepingService class has extra instrumentation (Yorick Peterse)
   - All service classes (those residing in app/services) are now instrumented (Yorick Peterse)
   - Developers can now add custom tags to transactions (Yorick Peterse)
   - Enable gzip for assets, makes the page size significantly smaller. !3544 / !3632 (Connor Shea)

app/services/projects/housekeeping_service.rb

@@ -26,22 +26,28 @@ module Projects
 
       GitlabShellOneShotWorker.perform_async(:gc, @project.path_with_namespace)
     ensure
+      Gitlab::Metrics.measure(:reset_pushes_since_gc) do
         @project.update_column(:pushes_since_gc, 0)
       end
+    end
 
     def needed?
       @project.pushes_since_gc >= 10
     end
 
     def increment!
+      Gitlab::Metrics.measure(:increment_pushes_since_gc) do
         @project.increment!(:pushes_since_gc)
       end
+    end
 
     private
 
     def try_obtain_lease
+      Gitlab::Metrics.measure(:obtain_housekeeping_lease) do
         lease = ::Gitlab::ExclusiveLease.new("project_housekeeping:#{@project.id}", timeout: LEASE_TIMEOUT)
         lease.try_obtain
       end
     end
+  end
 end