Revert "Remove Prometheus charm"

This reverts commit 722b5027

.

Reason for revert: Grafana-k8s requires Juju 3.1 and we are not ready
to change Juju versions at this time

Change-Id: Ibc17a6973c981a21152a8f74c8054e42ac21b24f
Signed-off-by: Mark Beierl <mark.beierl@canonical.com>