Add StatefulSet Start Ordinal metrics for KEP-3335

[pwschuurman]

What this PR does / why we need it:
Add metrics for new StatefulSet fields from KEP-3335

• KEP: Allow StatefulSet to control start replica ordinal numbering enhancements#3335

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

[CatherineF-dev]

QQ: how to set alerts around metric kube_statefulset_ordinals_start?

Does it compare with actual pod deletion/creation order?

[pwschuurman]

QQ: how to set alerts around metric kube_statefulset_ordinals_start?

This metric is used to inform a cluster administrator about usage, rather than correctness of the feature.

Does it compare with actual pod deletion/creation order?

kube_statefulset_ordinals_start reports the specified .spec.ordinals.start field, if .spec.ordinals is set. This enables a cluster administrator to monitor the usage of this feature. I considered adding a metric that indicated the status of the start ordinal, but in the KEP review, it was decided this wasn't super valuable to measure. See kubernetes/enhancements#3768 (comment) for context.

[pwschuurman]

/cc @soltysh

[soltysh]

/lgtm

[dgrisonnet]

/triage accepted
/assign @logicalhan

[dgrisonnet]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dgrisonnet, pwschuurman, soltysh

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