Added KubeProxyDown to runbook

Author: Andreas Kaminski

runbook.md

@@ -24,6 +24,10 @@ This page collects this repositories alerts and begins the process of describing
 ##### Alert Name: KubeletDown
 + *Message*: `Kubelet has disappeared from Prometheus target discovery.`
 + *Severity*: critical
+##### Alert Name: KubeProxyDown
++ *Message*: `KubeProxy has disappeared from Prometheus target discovery`
++ *Severity*: critical
++ *Runbook*: [Link](https://runbooks.prometheus-operator.dev/runbooks/kubernetes/kubeproxydown/)
 ### Group Name: kubernetes-apps
 ##### Alert Name: KubePodCrashLooping
 + *Message*: `{{ $labels.namespace }}/{{ $labels.pod }} ({{ $labels.container }}) is restarting {{ printf \"%.2f\" $value }} / second`