Feature request: UX enhancement: Introduce charm config option for Prometheus retention size

This is a feature request to help aid the Charm config user experience.

We've encountered the issue of Prometheus consuming too much disk space several times in our production environments.

The current solution is to limit how much disk space Prometheus can consume by setting a value for the flag --storage.tsdb.retention.size using the Charm's daemon-args config option (e.g. juju config prometheus daemon-args="--storage.tsdb.retention.size=20GB"). See: https://prometheus.io/docs/prometheus/latest/storage/#operational-aspects

However, unless you're familiar with Prometheus, this method is not immediately obvious.

The request is to expose the retention/size option as its own Charm config option (e.g. juju config prometheus retention-size=20GB).