code: @Override
    public FcmSendMessageResult sendMessageToRegistrationTokens(
        final Collection<String> registrationTokens,
        final FcmSendMessageRequest request
    ) {
      return proxy.sendMessageToRegistrationTokens(registrationTokens, request);
    }
docs: Sends an FCM message to the given set of registration tokens with the given request payload.
  
  @param registrationTokens the devices to send a message to.
  @param request            the details of the message.
  @return the result of sending the message.