Sensors which do not response for a long time are not marked as "unavailable"

[pointhi]

Describe the bug
When a sensor is no longer available (e.g. due to dead battery) Home Assistant is still reporing the current state as if it is a correct reading.

To Reproduce

1. Publish sensor reading
2. Remove e.g. battery from the sensor
3. Home assistant is showing the old reading even days later

Expected behavior

Expire sensor readings after some (configurable?) time when no new message arrives. See https://www.home-assistant.io/integrations/sensor.mqtt/#expire_after

Environment (please complete the following information):

• OpenMQTTGateway version used: v1.8.0

[NorthernMan54]

Openmqttgateway does not maintain state for any sensors sending data thru it. This is an issue for the application receiving the sensor data and not omg.

[pointhi]

The option expire_after I shared here is not a state Openmqttgateway needs to keep but an option which tells HA when to expire messages.

[Nealtron]

I believe this can be set via MQTT Discovery topics.

The included rtl_433_mqtt_hass.py auto discovery script provided by rtl_433 has a flag (-x) that sets a single value for the expiration of all data.

    parser.add_argument("-x", "--expire-after", type=int,
                        dest="expire_after",
                        help="Number of seconds with no updates after which the sensor becomes unavailable")

    if args.expire_after:
        config["expire_after"] = args.expire_after

    logging.debug(path + ":" + json.dumps(config))

    mqttc.publish(path, json.dumps(config), retain=args.retain)

    return True