client-go: Add informer metrics

[xigang]

Enhancement Description

• One-line enhancement description (can be used as a release note):
• Kubernetes Enhancement Proposal: https://github.com/kubernetes/enhancements/tree/master/keps/sig-api-machinery/4346-informer-metrics
• Discussion Link: client-go: Add metrics into Informer kubernetes#122067
• Primary contact (assignee): @xigang
• Responsible SIGs: sig/api-machinery
• Enhancement target (which target equals to which milestone):
  • Alpha release target (v1.33):
  • Beta release target (x.y):
  • Stable release target (x.y):
• Alpha
  • KEP (k/enhancements) update PR(s):
    • KEP-4346: Add Informer Metrics Initial KEP PR
  • Code (k/k) update PR(s):
    • KEP-4346: Add metrics for informer #129160
  • Docs (k/website) update PR(s):

Please keep this description up to date. This will help the Enhancement Team to track the evolution of the enhancement efficiently.

[xigang]

/assign @deads2k

[xigang]

These issues can be monitored through informer metrics. I think informer metrics are important indicators for the client.

kubernetes/kubernetes#121474
kubernetes/kubernetes#129795
kubernetes/kubernetes#117123
kubernetes/kubernetes#122067 (comment)
kubernetes/kubernetes#130767
kubernetes/client-go#1027
kubernetes-sigs/controller-runtime#817

[xigang]

/sig scalability
/sig api-machinery
/priority important-soon