Allow to send battery level to InfluxDB

[benhubert]

As far as I see, it's not possible at the moment to forward the battery level of a sensor node to InfluxDB. In the resources data settings I only see the sensor values, not the battery level.

One workaround is to send the battery level as additional multimeter sensor value. But this requires changes in the code and costs additional power.

Since the sensor nodes already send this data as I_BATTERY_LEVEL, I'd propose to allow forwarding this internal messages to external databases.

Related: https://forum.mycontroller.org/topic/379/send-battery-level-to-influxdb