Instrument MergeRequestDiff#load_commits

This instrumentation isn't needed strictly for performance measurements, but
just to see which controller actions call this method at all. See
<https://gitlab.com/gitlab-org/gitlab-ce/issues/30224#note_32306159> for more
details.

changelogs/unreleased/instrument-merge-request-diff-load-commits.yml

+---
+title: Instrument MergeRequestDiff#load_commits
+merge_request:
+author:

config/initializers/8_metrics.rb

@@ -113,6 +113,9 @@ def instrument_classes(instrumentation)
 
   # This is a Rails scope so we have to instrument it manually.
   instrumentation.instrument_method(Project, :visible_to_user)
+
+  # Needed for https://gitlab.com/gitlab-org/gitlab-ce/issues/30224#note_32306159
+  instrumentation.instrument_instance_method(MergeRequestDiff, :load_commits)
 end
 # rubocop:enable Metrics/AbcSize
 

spec/initializers/8_metrics_spec.rb

@@ -7,6 +7,7 @@ describe 'instrument_classes', lib: true do
   before do
     allow(config).to receive(:instrument_method)
     allow(config).to receive(:instrument_methods)
+    allow(config).to receive(:instrument_instance_method)
     allow(config).to receive(:instrument_instance_methods)
   end