Merge branch 'instrument-gitlab-highlight' into 'master'

Instrument Gitlab::Highlight

## What does this MR do?

This instruments the class `Gitlab::Highlight`.

## Are there points in the code the reviewer needs to double check?

No.

## Why was this MR needed?

This class is not instrumented.

## What are the relevant issue numbers?

#18592

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5644

CHANGELOG

@@ -35,6 +35,7 @@ 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