Instrument Gitlab::Highlight

This class does quite a few interesting things so let's instrument it so
we can see how much time is being spent in this class.

CHANGELOG

@@ -30,6 +30,7 @@ v 8.11.0 (unreleased)
   - Include old revision in merge request update hooks (Ben Boeckel)
   - Add build event color in HipChat messages (David Eisner)
   - Make fork counter always clickable. !5463 (winniehell)
+  - Gitlab::Highlight is now instrumented
   - All created issues, API or WebUI, can be submitted to Akismet for spam check !5333
   - The overhead of instrumented method calls has been reduced
   - Remove `search_id` of labels dropdown filter to fix 'Missleading URI for labels in Merge Requests and Issues view'. !5368 (Scott Le)

config/initializers/metrics.rb

@@ -145,6 +145,9 @@ if Gitlab::Metrics.enabled?
 
     config.instrument_methods(Rinku)
     config.instrument_instance_methods(Repository)
+
+    config.instrument_methods(Gitlab::Highlight)
+    config.instrument_instance_methods(Gitlab::Highlight)
   end
 
   GC::Profiler.enable