code: public void onHttpServerUpgrade(Http2Settings settings) throws Http2Exception {
          if (!connection().isServer()) {
              throw connectionError(PROTOCOL_ERROR, "Server-side HTTP upgrade requested for a client");
          }
          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");
          }
  
          // Apply the settings but no ACK is necessary.
          encoder.remoteSettings(settings);
  
          // Create a stream in the half-closed state.
          connection().remote().createStream(HTTP_UPGRADE_STREAM_ID, true);
      }
docs: Handles the server-side (cleartext) upgrade from HTTP to HTTP/2.
  @param settings the settings for the remote endpoint.