Support for JobSet metrics

[chelseychen]

What would you like to be added:

Expose metrics about JobSet resource: https://jobset.sigs.k8s.io/docs/reference/jobset.v1alpha2/

Why is this needed:

For better monitoring of AI training job status.

Describe the solution you'd like

Following docs/developer/guide.md to add a new JobSet resource with a few metrics reporting the status of the JobSet.

Additional context

[k8s-ci-robot]

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