kube-prometheus: add dead man's switch

manifests/prometheus/prometheus-k8s-rules.yaml

@@ -235,6 +235,18 @@ data:
         description = "A target of type {{ $labels.job }} is down."
       }
     
+    ### Dead man's switch ###
+    
+    ALERT DeadMansSwitch
+      IF vector(1)
+      LABELS {
+        severity = "none",
+      }
+      ANNOTATIONS {
+        summary = "Alerting DeadMansSwitch",
+        description = "This is a DeadMansSwitch meant to ensure that the entire Alerting pipeline is functional.",
+      }
+    
     ### File descriptor alerts ###
     
     ALERT TooManyOpenFiles