Displays accurate request queue times for New Relic on Heroku Cedar

gistfile1.rb

# Accurate queue times for Heroku Cedar + Rails (or Rack)
#
# For Rails:
#
#     Rails::Application.middleware.insert(0, QueueTimeLogger)
#
class QueueTimeLogger
  attr_reader :app

  def initialize(app, options = {})
    @app = app
  end

  def call(env)
    now_us = (Time.now.to_f * 1000000).to_i
    request_start_us = env["HTTP_X_REQUEST_START"].to_i * 1000

    queue_time_us = now_us - request_start_us
    env["HTTP_X_QUEUE_TIME"] = "t=#{queue_time_us}"

    # This overrides HTTP_X_QUEUE_TIME and doesn't provide anything meaningful.
    env.delete("HTTP_X_HEROKU_QUEUE_WAIT_TIME")

    app.call(env)
  end
end