New metric: kube_crd_created

[alshain]

What would you like to be added:

It seems currently impossible to track the creation timestamp of objects of type CustomResourceDefinition.
I'd like for KSM to expose kube_crd_created by default.

Why is this needed:

With operators, we've had hickups where the CRD object itself got deleted momentarily and all instances were lost until

Describe the solution you'd like:

I'd like for KSM to expose kube_crd_created by default.

Additional context:

[kubernetes-prow]

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Details

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[emmayusufu]

I can take this one. It needs a new store for CustomResourceDefinition objects plus the apiextensions client wired into the builder, so a bit more than the usual add-a-metric change, but the shape is the same as the other _created timestamps. If the maintainers are open to it I'll put up a PR.

/assign