expose config option to enable admin API

[baycarbone]

Enhancement Proposal

In order to take snapshots of the prometheus data, when performing backups for example, it is necessary to interact with the admin API.
However, admin API is disabled by default and it is not possible to enable it as the charm does not expose such a config option today.

reference: https://devopstales.github.io/kubernetes/backup-and-retore-prometheus/