docs: public void calculate(ServiceMessage message) {
      // client to service
      eval(
          message.header(SERVICE_RECV_TIME),
          message.header(CLIENT_SEND_TIME),
          (v1, v2) -> clientToServiceTimer.update(v1 - v2, TimeUnit.MILLISECONDS));
  
      // service to client
      eval(
          message.header(CLIENT_RECV_TIME),
          message.header(SERVICE_SEND_TIME),
          (v1, v2) -> serviceToClientTimer.update(v1 - v2, TimeUnit.MILLISECONDS));
    }