Provide the time when a pod is initialized

[opeco17]

What would you like to be added:

It would be great to have the time when a pod is initialized in the lifecycle.

Why is this needed:

I'm hosting pods that take a long time to complete startup probe, and I would like to know how long it takes accurately for performance tuning.

To calculate that period, it's necessary to know when a pod is initialized and ready.

Even though the latter one is already implemented (kube_pod_status_ready_time), the former one is not yet.

Describe the solution you'd like

Add the new metric that describes the time when a pod is initialized

PR: #2148

Additional context

[k8s-ci-robot]

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