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/erratic/health.go

@@ -0,0 +1,14 @@
+package erratic
+
+import (
+	"sync/atomic"
+)
+
+// Health implements the health.Healther interface.
+func (e *Erratic) Health() bool {
+	q := atomic.LoadUint64(&e.q)
+	if e.drop > 0 && q%e.drop == 0 {
+		return false
+	}
+	return true
+}