Disable Instrumentation load under FF

config/feature_flags/development/method_instrumentation_disable_initialization.yml

+---
+name: method_instrumentation_disable_initialization
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/69091
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/339665
+milestone: '14.3'
+type: development
+group: group::memory
+default_enabled: false

config/initializers/zz_metrics.rb

@@ -178,6 +178,9 @@ if Gitlab::Metrics.enabled? && !Rails.env.test? && !(Rails.env.development? && d
       ActiveRecord::Querying.public_instance_methods(false).map(&:to_s)
     )
 
+    # We are removing the Instrumentation module entirely in steps.
+    # More in https://gitlab.com/gitlab-org/gitlab/-/issues/217978.
+    unless ::Feature.enabled?(:method_instrumentation_disable_initialization)
       Gitlab::Metrics::Instrumentation
         .instrument_class_hierarchy(ActiveRecord::Base) do |klass, method|
           # Instrumenting the ApplicationSetting class can lead to an infinite
@@ -196,10 +199,13 @@ if Gitlab::Metrics.enabled? && !Rails.env.test? && !(Rails.env.development? && d
       # be instrumented.
       Gitlab::Metrics::Instrumentation.instrument_methods(Ability)
     end
+  end
 
+  unless ::Feature.enabled?(:method_instrumentation_disable_initialization)
     Gitlab::Metrics::Instrumentation.configure do |config|
       instrument_classes(config)
     end
+  end
 
   GC::Profiler.enable