Revert "Remove Grafana charm"

This reverts commit 027439f8.

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

Change-Id: I2f1056da47a9c0afc63fc2f547dbebbf667d11e6
Signed-off-by: Mark Beierl <mark.beierl@canonical.com>