docs: public void addShutdownListener(final ShutdownListener shutdownListener) {
          synchronized (lock) {
              if (!shutdown) {
                  throw UndertowMessages.MESSAGES.handlerNotShutdown();
              }
              long count = activeRequestsUpdater.get(this);
              if (count == 0) {
                  shutdownListener.shutdown(true);
              } else {
                  shutdownListeners.add(shutdownListener);
              }
          }
      }