Adds link_to_gfm method instrumentation

changelogs/unreleased/add-instrumentation-to-link-to-gfm.yml

+---
+title: Add instrumentation to MarkupHelper#link_to_gfm
+merge_request: 13069
+author:

config/initializers/8_metrics.rb

@@ -114,6 +114,9 @@ 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/34509
+  instrumentation.instrument_method(MarkupHelper, :link_to_gfm)
+
   # Needed for https://gitlab.com/gitlab-org/gitlab-ce/issues/30224#note_32306159
   instrumentation.instrument_instance_method(MergeRequestDiff, :load_commits)
 end