Add `SchedulingGated` reason to `kube_pod_status_reason` metric

[jjacobelli]

What would you like to be added:

Currently, there is no metrics exposing if a pod is in Pending phase because it is SchedulingGated. Having kube_pod_status_reason exposing this would help to write better alerting rules to prevent sending alerts for pending pods if they are just in SchedulingGated phase.

Why is this needed:

Preventing alert rules to trigger on Pending pods that are in SchedulingGated state

Describe the solution you'd like:

Add SchedulingGated reason to the kube_pod_status_reason metrics

Additional context:

I can open a PR to fix this issue if that can help.

[k8s-ci-robot]

This issue is currently awaiting triage.

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