Disable method instrumentation for diffs

When there a large number of discussions on diffs, the instrumentation
has a significant impact on performance since it's often run in a tight
loop for line position calculation etc. We should disable this since
there are other ways to benchmark performance.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52898

changelogs/unreleased/sh-disable-diff-instrumentation.yml

+---
+title: Disable method instrumentation for diffs
+merge_request: 27235
+author:
+type: performance

config/initializers/zz_metrics.rb

@@ -30,7 +30,6 @@ def instrument_classes(instrumentation)
     # are included.
     %w(app services [^concerns]**) => %w(app services),
     %w(lib gitlab conflicts)       => ['lib'],
-    %w(lib gitlab diff)            => ['lib'],
     %w(lib gitlab email message)   => ['lib'],
     %w(lib gitlab checks)          => ['lib']
   }