Fix KubernetesStatefulsetDown prom rule

kube-system/kube-state-metrics.yaml

@@ -319,8 +319,11 @@ spec:
             summary: Kubernetes PersistentVolume error (instance {{ $labels.instance }})
             description: "Persistent volume is in bad state\n  VALUE = {{ $value }}\n  LABELS = {{ $labels }}"
         - alert: KubernetesStatefulsetDown
-          expr: (kube_statefulset_status_replicas_ready / kube_statefulset_status_replicas_current) != 1
-          for: 1m
+          # compare against spec replicas: status_replicas_current is 0 on
+          # healthy statefulsets after rollouts (false positives), and a
+          # ready/desired division breaks on scale-to-zero (0/0 = NaN)
+          expr: kube_statefulset_status_replicas_ready < kube_statefulset_replicas
+          for: 5m
           labels:
             severity: critical
           annotations: