Add feature flag definition and controller check

app/controllers/admin/integrations_controller.rb

@@ -5,6 +5,7 @@ class Admin::IntegrationsController < Admin::ApplicationController
   include IntegrationsHelper
 
   before_action :not_found, unless: -> { instance_level_integrations? }
+  before_action :not_found, unless: -> { instance_level_integration_overrides? }, only: :overrides
 
   feature_category :integrations
 
@@ -25,4 +26,8 @@ class Admin::IntegrationsController < Admin::ApplicationController
   def find_or_initialize_non_project_specific_integration(name)
     Integration.find_or_initialize_non_project_specific_integration(name, instance: true)
   end
+
+  def instance_level_integration_overrides?
+    Feature.enabled?(:instance_level_integration_overrides, default_enabled: :yaml)
+  end
 end

config/feature_flags/development/instance_level_integration_overrides.yml

+---
+name: instance_level_integration_overrides
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66723
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/336750
+milestone: '14.2'
+type: development
+group: group::ecosystem
+default_enabled: false