Introduce time-based loadpoint metrics

[naltatis]

Since this is a topic that comes up regularly, I'm creating this issue here to funnel requests.

Currently, we collect a lot of data (energy, % solar, price, co2, meter readings, ...) based on charging sessions. Since evcc's primary use case is EV charging, this is perfectly fine and will remain as is. However, the session concept does not really work for use-cases where no device is plugged in/out (heat pump, water heater, cooler, ...) and for chargers where we can't detect the connection status (smart plugs). There are workarounds to produce logged charging sessions in these scenarios (temporary PV/Stop/PV, service restart), but these are ugly.

A better solution is to also collect and store the above values on a pure time basis. This way can build hourly/daily/monthly statistics for every kind of load point. We'll probably not do high-resolution data collection (please use something like InfluxDB/Grafana for this), but enough resolution to create meaningful visualizations.

This topic is not very high on our priority list right now, but we'll build something like this eventually.