code: @Override
    public FcmSendMessageResult sendMessageTo(final String to, final FcmSendMessageRequest request) {
      return proxy.sendMessageTo(to, request);
    }
docs: Sends an FCM message to the given target with the given request payload.
  
  @param to      the target to send a message to.
  @param request the details of the message.
  @return the result of sending the message.