+ prometheus: add configuration parameters to the accumulator

[Iilun]

PR Goal

As mentioned in #777, and some discord conversations, the issue of prometheus keeping removed metrics for 5 years can sometimes cause issues.

Choices

The current configuration behavior does not handle reconfiguration. I did not want to make choices on whether to keep data or not if a reload had to modify the snapshot accumulator, and unsure if making the accumulator variable was a good idea anyway. I am of course open to feedback on this issue

[mkurz]

@ivantopo this one please a well, thanks!

[Iilun]

@ivantopo Is it possible to take a look at this ? Thank you very much !