feat(pod): Add new metric for pod unscheduled time tracking

[yshngg]

What this PR does / why we need it:

The changes introduce a new metric generator kube_pod_status_unscheduled_time to record the Unix timestamp when pods transition to unscheduled status.

How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)

increases

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):

Fixes #2698

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[CatherineF-dev]

Could you share use cases after adding this metric kube_pod_status_unscheduled_time?

[yshngg]

Hi @CatherineF-dev,

In my use case, this metric enables proactive scheduling failure monitoring by supporting alerting workflows (e.g., triggering alerts when Pods remain unschedulable > 5 minutes to prevent service degradation).