Node removal latency metrics added

[ttetyanka]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR introduces a new histogram metric cluster_autoscaler_node_deletion_duration_seconds that tracks the time a node spends marked as unneeded before it is either:

deleted (label deleted="true")

or becomes needed again without deletion (label deleted="false")

The observed duration is adjusted by subtracting the configured scale-down threshold to better reflect actual decision latency.

The metric is guarded by a feature flag and is disabled by default.

This adds visibility into scale-down behavior, allowing operators to understand both successful and aborted scale-downs.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Introduces NodeLatencyTracker for tracking unneeded nodes.

Records both successful and aborted deletions, with threshold adjustment.

Feature-flagged; no impact unless explicitly enabled.

Includes unit tests covering core scenarios.

Does this PR introduce a user-facing change?

Cluster Autoscaler adds a new Prometheus histogram metric (behind a feature flag):
`cluster_autoscaler_node_deletion_duration_seconds` — duration from when a node is marked as unneeded until it is either deleted (`deleted="true"`) or becomes needed again (`deleted="false"`).  
Reported values are adjusted by subtracting the configured scale-down threshold.

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

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[linux-foundation-easycla]

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• ✅ login: ttetyanka / name: Tetiana Yeremenko (697d57e, 3bd35bf, 905cb29, ba29eef, 4902e0f, 5058928, 09bb88d, 191c494, 045e739, 7512a0d, 6966d04, 97eb2f5, 71be229, 90180f0, 5702fde, 32c7b57, 445bc9f, 75249a6, bb064bd)

[k8s-ci-robot]

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[k8s-ci-robot]

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[elmiko]

/ok-to-test

[MartynaGrotek]

Planner and actuator get different instances, it wouldn’t work correctly if tests cover that. I would pass null or the same instance.

[ttetyanka]

used nil in all the other tests, but in TestStaticAutoscalerRunOnce added a mocked instance and verified the method calls

[MartynaGrotek]

Is it even possible? I would expect that ObserveDeletion will remove these nodes before.
If it is possible, we are not reporting such node at all.

[ttetyanka]

This is possible in the following scenario: for example, a node was marked as unneeded during one autoscaler loop, and we start tracking it. Then, it cannot be deleted because the minimum node pool size is reached, so the node is not deleted and is no longer marked as unneeded. Therefore, we have to remove it from observation.

So we don’t want to report these cases, just silently remove them?