code: @Override
      public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
          if (evt instanceof HttpServerUpgradeHandler.UpgradeEvent) {
              HttpServerUpgradeHandler.UpgradeEvent upgradeEvent =
                      (HttpServerUpgradeHandler.UpgradeEvent) evt;
              onHeadersRead(ctx, 1, http1HeadersToHttp2Headers(upgradeEvent.upgradeRequest()), 0 , true);
          }
          super.userEventTriggered(ctx, evt);
      }
docs: Handles the cleartext HTTP upgrade event. If an upgrade occurred, sends a simple response via HTTP/2
  on stream 1 (the stream specifically reserved for cleartext HTTP upgrade).