code: public static ModelControllerClient createAndAdd(final ManagementChannelHandler handler) {
          final ExistingChannelModelControllerClient client = new ExistingChannelModelControllerClient(handler);
          handler.addHandlerFactory(client);
          return client;
      }
docs: Create and add model controller handler to an existing management channel handler.
  
  @param handler the channel handler
  @return the created client