Add a new Key metric

[ribalba]

Currently we display some metrics in our Key metrics boxes. Things like CO2 or Energy might be quite abstract and not as quantifiable for may people. I have no idea if 2kg of CO2 is a lot or not. We try to visualise this in the Detailed metrics with Trees etc ...

I propose a new Key metric "price". We know fairly well how much energy costs for the average consumer. Obviously it depends heavily where the code is run but I think it will make it a lot clearer and we can always add it as a configurable variable.

https://www.destatis.de/DE/Themen/Wirtschaft/Preise/Erdgas-Strom-DurchschnittsPreise/_inhalt.html

Came up in a discussion with Kira