Merge branch '38930-warmup' into 'master'

Resolve "First request after GDK server started is slow"

Closes #38930

See merge request gitlab-org/gitlab-ce!16153

config.ru

@@ -17,6 +17,11 @@ end
 
 require ::File.expand_path('../config/environment',  __FILE__)
 
+warmup do |app|
+  client = Rack::MockRequest.new(app)
+  client.get('/')
+end
+
 map ENV['RAILS_RELATIVE_URL_ROOT'] || "/" do
   run Gitlab::Application
 end