monitoring: make cluster matcher backward compatible for pre-7.1 metrics

monitoring: make cluster matcher backward compatible for pre-7.1 metrics

Ceph 18.* adds a `cluster` label to all Prometheus metrics. When
upgrading from earlier releases, historical metrics lack this label
and are excluded by Grafana queries that strictly match on `cluster`.
Update the shared Grafana matcher logic to use a regex matcher that
also matches series without the `cluster` label, restoring visibility
of pre-upgrade metrics while preserving multi-cluster behavior.

Fixes: https://tracker.ceph.com/issues/74342
Signed-off-by: Aashish Sharma <aasharma@redhat.com>