Developer Mode is now a Rack middleware.
Developer Mode is no longer available in Rails 2.1 and earlier. However, starting in version 2.12 you can use Developer Mode in any Rack based framework, in addition to Rails. To install developer mode in a non-Rails application, just add NewRelic::Rack::DeveloperMode to your middleware stack.
If you are using JRuby, we recommend using at least version 1.4 or later because of issues with the implementation of the timeout library.
Refer to the README.md file for more information.