Prometheus total Energy export

[Matyro]

Is your feature request related to a problem? Please describe.
Prometheus exporter does only provide current power and energy consumption between each push. As far as I know the prometheus push gateway only caches a single values. Therefore if prometheus querys not often enough it is possible to miss values.

Describe the solution you'd like
Export the total energy since start of the application

[cianc]

@inimaz if you'd like I can take this. I think it should be a fairly straightforward addition of a counter.

[Matyro]

Not sure if this was the best solution but I implemented this in MR #984