metal.rb

require 'sinatra'

module Sinatra::Metal
  module ClassMethods
    def metal(base = nil)
      base = '' unless base || @metal
      @metal = /^#{Regexp.escape base}/ if base
      @metal
    end
  end

  def self.included(base)
    base.extend ClassMethods
    base.disable :raise_errors
    base.app_file caller.first.split(':').first
    base.before { not_found unless request.path_info =~ self.class.metal }
    base.metal
  end
end

sinatra_metal.rb

require 'sinatra/metal'

class SinatraMetal < Sinatra::Base
  include Sinatra::Metal

  get '/sinatra' do
    'hello sinatra!'
  end
end