fix(monitoring): Limit prometheus retention by disk size

This patch adjusts the retention to be either 15 days or 15GB of
straoge, which should help to keep data growth under control.

It might be an idea, to prodive a more longterm metrics storage later
on for a selected subset.

The patch also adjust the pvc size once more to account of the size
settings and makes sure there are 2 GiB of additional headspace.

References:
https://prometheus.io/docs/prometheus/latest/storage/#operational-aspects