docs: public HttpServerExchange upgradeChannel(String productName, final HttpUpgradeListener listener) {
          if (!connection.isUpgradeSupported()) {
              throw UndertowMessages.MESSAGES.upgradeNotSupported();
          }
          UndertowLogger.REQUEST_LOGGER.debugf("Upgrading request %s", this);
          connection.setUpgradeListener(listener);
          setStatusCode(StatusCodes.SWITCHING_PROTOCOLS);
          final HeaderMap headers = getResponseHeaders();
          headers.put(Headers.UPGRADE, productName);
          headers.put(Headers.CONNECTION, Headers.UPGRADE_STRING);
          return this;
      }