Digest auth with Rack/Metal

Hi folks,

I'm trying to so some digest authentication in Rack via Metal. I've
found the:

  http://rack.rubyforge.org/doc/Rack/Auth/Digest/MD5.html

Class, but I don't have a great idea of how this fits into metal. It
seems like I should be able to do something similar to:

  class MyAuth < Rack::Auth::Digest::MD5
  
    def initialize(app, realm=nil)
      super(app, (realm or "Awesome Auth")) do |username,password|
        username == 'bob' and password == 'jimbo'
      end
    end
  
  end

Any thoughts or experience using Rack::Auth::Digest::MD5? I think I
could just instantiate it on each request, but it's built *like* a rack
application so I don't see why that's needed.

Thanks all,