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>
(cherry picked from commit f7f74e455423feab79b33ac8ab472be0b98cb29d)

 Conflicts:
	monitoring/ceph-mixin/dashboards_out/ceph-application-overview.json (file not in squid)
	monitoring/ceph-mixin/dashboards_out/ceph-cluster-advanced.json
(conflicts with $rate_interval in main)
	monitoring/ceph-mixin/dashboards_out/ceph-cluster.json (missing
cluster label in metrics)
	monitoring/ceph-mixin/dashboards_out/cephfsdashboard.json (file
not in squid)
	monitoring/ceph-mixin/dashboards_out/multi-cluster-overview.json
(file not in squid)