feat: expose NotebookCullingCount and NotebookCullingTimestamp in prometheus metrics

[henrysecond1]

Currently, notebook culling metrics are not exported.

This PR fixes two things

• Export culling metrics
• Remove duplicated increment of notebook_culling_total (incremented twice in SetStopAnnotation and Reconcile)

[aws-kf-ci-bot]

Hi @henrysecond1. Thanks for your PR.

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[google-oss-prow]

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[Arhell]

/ok-to-test

[juliusvonkohout]

is this something you want @kubeflow/wg-notebooks-leads ?

[juliusvonkohout]

Can you join the next notebooks WG meeting in two weeks ? We are always looking for help.

https://www.kubeflow.org/docs/about/community/#kubeflow-community-calendars has the calendar and meeting notes.

You can also reach us on slack in the channel wg-notebooks and wg-manifests.