Disable method instrumentation initialization

We are deprecating the method instrumentation the way we implemented it,
as we are not using the data produced by the instrumented classes and
methods while the approach:
- redefines every method with metaprogramming/monkey patching
- makes some assumptions on the method arity which is tricky to test
and may be prone to errors
- appears almost on every trace, so it could mask the real issue
with the argument mismatch
- does something that the profiler should do,
but with the custom solution
- adds performance & memory penalty to our call chain
The implementation itself would be removed in next MRs, to have smaller
iterations and reduce the risks.

Changelog: removed

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

@@ -162,49 +162,6 @@ if Gitlab::Metrics.enabled? && !Rails.env.test? && !(Rails.env.development? && d
     config.middleware.use(Gitlab::Metrics::ElasticsearchRackMiddleware)
   end
 
-  # 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)
-    # This instruments all methods residing in app/models that (appear to) use any
-    # of the ActiveRecord methods. This has to take place _after_ initializing as
-    # for some unknown reason calling eager_load! earlier breaks Devise.
-    Gitlab::Application.config.after_initialize do
-      # We should move all the logic of this file to somewhere else
-      # and require it after `Rails.application.initialize!` in `environment.rb` file.
-      models_path = Rails.root.join('app', 'models').to_s
-
-      Dir.glob("**/*.rb", base: models_path).sort.each do |file|
-        require_dependency file
-      end
-
-      regex = Regexp.union(
-        ActiveRecord::Querying.public_instance_methods(false).map(&:to_s)
-      )
-
-      Gitlab::Metrics::Instrumentation
-        .instrument_class_hierarchy(ActiveRecord::Base) do |klass, method|
-          # Instrumenting the ApplicationSetting class can lead to an infinite
-          # loop. Since the data is cached any way we don't really need to
-          # instrument it.
-          if klass == ApplicationSetting
-            false
-          else
-            loc = method.source_location
-
-            loc && loc[0].start_with?(models_path) && method.source =~ regex
-          end
-        end
-
-      # Ability is in app/models, is not an ActiveRecord model, but should still
-      # be instrumented.
-      Gitlab::Metrics::Instrumentation.instrument_methods(Ability)
-    end
-
-    Gitlab::Metrics::Instrumentation.configure do |config|
-      instrument_classes(config)
-    end
-  end
-
   GC::Profiler.enable
 
   module TrackNewRedisConnections