middleware.rb

class RequestQueueTimeMiddleware
  MILLISECONDS_CUTOFF = Time.new(2000, 1, 1).to_i * 1000
  MICROSECONDS_CUTOFF = MILLISECONDS_CUTOFF * 1000
  NANOSECONDS_CUTOFF = MICROSECONDS_CUTOFF * 1000

  def initialize(app)
    @app = app
  end

  def call(env)
    queue_time = calculate_queue_time(env)

    status, headers, body = @app.call(env)

    if queue_time
      # Replace this with your APM agent
      NewRelic::Agent.add_custom_attributes({request_queue_time: queue_time})
    end

    [status, headers, body]
  end

  def calculate_queue_time(env)
    request_start = parse_request_start(env["HTTP_X_REQUEST_START"])
    return nil unless request_start

    now = Time.zone.now
    queue_time = ((now - request_start) * 1000).to_i

    queue_time -= env["puma.request_body_wait"].to_i

    (queue_time > 0) ? queue_time : 0
  end

  def parse_request_start(header)
    return nil unless header

    value = header.gsub(/[^0-9.]/, "").to_f

    if value > NANOSECONDS_CUTOFF
      Time.zone.at(value / 1_000_000_000.0)
    elsif value > MICROSECONDS_CUTOFF
      Time.zone.at(value / 1_000_000.0)
    elsif value > MILLISECONDS_CUTOFF
      Time.zone.at(value / 1000.0)
    else
      Time.zone.at(value)
    end
  end
end