function publish(topic, message) {
  if (!this.mqttClient) {
    throw new ServiceNotConfiguredError('MQTT is not configured.');
  }
  logger.debug(`Publishing MQTT message on topic ${topic}`);
  this.mqttClient.publish(topic, message, undefined, (err) => {
    if (err) {
      logger.warn(`MQTT - Error publishing to ${topic} : ${err}`);
    }
  });
}
@description Publish a MQTT message.
@param {string} topic - MQTT Topic.
@param {string} message - MQTT message.
@example zigbee2mqtt.publish('zigbee2mqtt/test', '{}');