title: The Rails Metal in Ruby on Rails 3
body: <p>I am using Ruby on Rails 3 and I would like to implement some Rack middleware. </p>
  
  <p>I know (but maybe I am wrong!) that, before of version 3, there was the Rails Metal "system" to handle those. But now?!</p>
  
  <p><strong>Where in my Rails application I have to locate files to use for middleware purposes</strong> (before of version 3, if I am not wrong, the folder was named 'metal')<strong>? then, how I must state them in the <code>application.rb</code> file?</strong></p>