Merge branch 'instrument-nokogiri' into 'master'

Instrument Nokogiri parsing methods

## What does this MR do?

This MR instruments the various parsing methods of Nokogiri.

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

No.

## Why was this MR needed?

This allows us to see how much time is spent in Nokogiri.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] ~~Added for this feature/bug~~
  - [ ] All builds are passing
- [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 !5470

CHANGELOG

@@ -5,6 +5,7 @@ v 8.11.0 (unreleased)
   - Fix of 'Commits being passed to custom hooks are already reachable when using the UI'
   - Limit git rev-list output count to one in forced push check
   - Retrieve rendered HTML from cache in one request
+  - Nokogiri's various parsing methods are now instrumented
   - Make fork counter always clickable !5463 (winniehell)
   - Load project invited groups and members eagerly in ProjectTeam#fetch_members
   - Add GitLab Workhorse version to admin dashboard (Katarzyna Kobierska Ula Budziszewska)

config/initializers/metrics.rb

@@ -136,6 +136,13 @@ if Gitlab::Metrics.enabled?
     config.instrument_instance_methods(Rouge::Plugins::Redcarpet)
     config.instrument_instance_methods(Rouge::Formatters::HTMLGitlab)
 
+    [:XML, :HTML].each do |namespace|
+      namespace_mod = Nokogiri.const_get(namespace)
+
+      config.instrument_methods(namespace_mod)
+      config.instrument_methods(namespace_mod::Document)
+    end
+
     config.instrument_methods(Rinku)
   end