kube_pod_status_reason metric add Preempting reason

[zou-weidong]

What would you like to be added:
Add ‘Preempting ’ reason to kube_pod_status_reason

Why is this needed:
We have an alert for pods that are not ready for a long time(Use prometheus-operator by default to have the alert KubePodNotReady ). I want to eliminate some situations, including Evicted and Preempting. However, there is no Preempting in the current reason.

Describe the solution you'd like

kube_pod_status_reason | Gauge | The pod status reasons | 
reason=<Evicted|NodeAffinity|NodeLost|Shutdown|UnexpectedAdmissionError>

Add Preempting

The kubernetes code:

[k8s-ci-robot]

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