Add State of Charge to PowerMetrics

[bergie]

Now power metrics can supply a set of voltage and current measurements. For devices where SoC can be measured, it would be nice to be able to supply it. Example where we'd love to be able to send it is signalk-meshtastic

This should probably follow the same convention as in DeviceMetrics:

// 0-100 (>100 means powered)
optional uint32 battery_level = 1;