Instrument Nokogiri parsing methods

This allows us to see how much time is being spent in just parsing
HTML/XML documents.

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