Time rack requests and print heroku request id

timer.rb

class RackTimer
  def initialize(app)
    @app = app
  end

  def call(env)
    start_request = Time.now
    status, headers, body = @app.call(env)
    elapsed = (Time.now - start_request) * 1000
    $stdout.puts("request-id=#{env['HTTP_HEROKU_REQUEST_ID']} measure.rack-request=#{elapsed.round}ms")
    [status, headers, body]
  end
end

Would you recommended an expanded version like: https://gist.github.com/chinshr/5314155

In either case, where exactly would I put it?