KubeAPIDown alert rename and/or degraded status

The alert KubeAPIDown is not reflecting what really the alert means. 

In the summary, it's indicated that 

  "Target disappeared from Prometheus target discovery"

but really, the only values are 0 or 1 for this alert, then, if you bring down a kubeapi server, this alert is not triggered.

4.13 and previous

Always. Shutting down a master node

1. Shutdown a master node
2. Wait and check the alert KubeAPIDown never is triggered since it's 1 or 2 and 2 apiserver are up. 

The alert is not reflecting the summary:

  "Target disappeared from Prometheus target discovery"

The alert is triggered if we respect the summary and the alertname. Then 2 possible options that they could go together:

1. Create an alert that reflects the status of the kubeapi server pods. Then, a apiserver pod is down, it's reflected as a degraded status
2. Modify the current alert to the name summary to reflect the really monitored