Rack::RequestTimer

config.ru

require "time"

module Rack

  class RequestTimer

    class Timer

      FORMAT = "%0.3fms elapsed since %s" unless defined?(FORMAT)

      attr_reader :start_time

      def initialize
        @start_time = Time.now
      end

      def elapsed_seconds
        Time.now.to_i - @start_time.to_i
      end

      def elapsed_milliseconds
        (Time.now.to_f - @start_time.to_f) * 1000
      end

      def to_s
        format(FORMAT, elapsed_milliseconds, @start_time)
      end

    end

    ENV_VARIABLE = "rack.request_timer" unless defined?(ENV_VARIABLE)

    def initialize(app, env_variable: ENV_VARIABLE)
      @app = app
      @env_variable = env_variable
    end

    def call(env)
      env[@env_variable] = Timer.new
      @app.call(env)
    end

    def self.timer(env)
      env[ENV_VARIABLE]
    end

    def self.elapsed_seconds(env)
      env[ENV_VARIABLE].elapsed_seconds if env[ENV_VARIABLE]
    end

    def self.elapsed_milliseconds(env)
      env[ENV_VARIABLE].elapsed_milliseconds if env[ENV_VARIABLE]
    end

  end

end

app = ->(env) {
  sleep(rand)
  [200, {"Content-Type" => "text/plain"}, ["OK: response time: #{Rack::RequestTimer::timer(env)}"]]
}

use Rack::RequestTimer
run app