plugin/health: implement dyn health checks (#1214)

Implement health.Healther in erratic and kubernetes plugin. The
kubernetes' healtcheck is only performed on startup - i.e. turn
healthy after the initial loading.

Erratic follow the drop count: every query%drop turns the healthcheck
unhealthy.

Fixes: #985

plugin/kubernetes/README.md

@@ -85,6 +85,11 @@ kubernetes [ZONES...] {
   what the response will be. However, if you specify this option, the query will instead be passed
   on down the plugin chain, which can include another plugin to handle the query.
 
+## Health
+
+This plugin implements dynamic health checking. Currently this is limited to reporting healthy when
+the API has synced.
+
 ## Examples
 
 Handle all queries in the `cluster.local` zone. Connect to Kubernetes in-cluster. Also handle all

plugin/kubernetes/health.go

@@ -0,0 +1,4 @@
+package kubernetes
+
+// Health implements the health.Healther interface.
+func (k *Kubernetes) Health() bool { return k.APIConn.HasSynced() }