Kubelet: record the initial pod processing latency

Add a new latency metric for the time from seeing the pod for the first time
to starting a pod worker for it.

Also, change PodStartLatency to include this initial processing latency.

pkg/kubelet/types.go

@@ -24,6 +24,7 @@ import (
 
 const ConfigSourceAnnotationKey = "kubernetes.io/config.source"
 const ConfigMirrorAnnotationKey = "kubernetes.io/config.mirror"
+const ConfigFirstSeenAnnotationKey = "kubernetes.io/config.seen"
 
 // PodOperation defines what changes will be made on a pod configuration.
 type PodOperation int