code: public void onHttpClientUpgrade() throws Http2Exception {
          if (connection().isServer()) {
              throw connectionError(PROTOCOL_ERROR, "Client-side HTTP upgrade requested for a server");
          }
          if (!prefaceSent()) {
              // If the preface was not sent yet it most likely means the handler was not added to the pipeline before
              // calling this method.
              throw connectionError(INTERNAL_ERROR, "HTTP upgrade must occur after preface was sent");
          }
          if (decoder.prefaceReceived()) {
              throw connectionError(PROTOCOL_ERROR, "HTTP upgrade must occur before HTTP/2 preface is received");
          }
  
          // Create a local stream used for the HTTP cleartext upgrade.
          connection().local().createStream(HTTP_UPGRADE_STREAM_ID, true);
      }
docs: Handles the client-side (cleartext) upgrade from HTTP to HTTP/2.
  Reserves local stream 1 for the HTTP/2 response.