Enable Multi Cluster alerts by default (#2099)

manifests/kubePrometheus-prometheusRule.yaml

@@ -18,7 +18,7 @@ spec:
         description: '{{ printf "%.4g" $value }}% of the {{ $labels.job }}/{{ $labels.service }} targets in {{ $labels.namespace }} namespace are down.'
         runbook_url: https://runbooks.prometheus-operator.dev/runbooks/general/targetdown
         summary: One or more targets are unreachable.
-      expr: 100 * (count(up == 0) BY (job, namespace, service) / count(up) BY (job, namespace, service)) > 10
+      expr: 100 * (count(up == 0) BY (cluster, job, namespace, service) / count(up) BY (cluster, job, namespace, service)) > 10
       for: 10m
       labels:
         severity: warning