feat(nodeScaleDownTime): add a new metric to track unprocessed nodes during scaleDown

[shaikenov]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR adds a new metric to the exported prometheus metrics list: LongestNodeScaleDownTime

We want to track all the nodes that were marked as unneeded, but were unprocessed during the ScaleDown. If a node was unneeded, but unprocessed multiple times consecutively, we store only the earliest time it happened. The difference between the current time and the earliest time among all unprocessed nodes will give the longest time. This time can give us an indication of possible throttling and helps to better monitor what happens during ScaleDown.

Which issue(s) this PR fixes:

None

Special notes for your reviewer: this is a draft PR, work is still in progress

Does this PR introduce a user-facing change?

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

None

[k8s-ci-robot]

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[linux-foundation-easycla]

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• ✅ login: shaikenov / name: Olzhas Shaikenov (86a98d1)

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[k8s-ci-robot]

Hi @shaikenov. Thanks for your PR.

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[kada2004]

very good

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: kada2004, shaikenov
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