Support for the metrics with NotBefore timestamp

[Woytek-Polnik]

Could it be possible to add support to extra metrics which would say since when certficiate can be used
It should be fairly simple .NotBefore from https://pkg.go.dev/crypto/x509#pkg-variables

I've never touched go - and it would be super cool to have such metrics.

It would literally allow to compare NotBefore from k8s secret with the time when pod was started. And automatically restart pods when they were spawned before this cert (so cert was renewed)

[joe-elliott]

I would definitely merge this if someone has time to work on it.