subscribe method
Implementation
Future<Subscription> subscribe(
String key,
String channel, {
int last = 0,
}) async {
// var options = new Map<String, String>();
// if (last > 0) options["last"] = last.toString();
var topic = _formatChannel(key, channel, {});
return _mqtt!.subscribe(topic, MqttQos.atLeastOnce)!;
}