Graduate `custom.metrics.k8s.io` to stable

[rexagod]

Enhancement Description

• One-line enhancement description (can be used as a release note): Graduate custom.metrics.k8s.io API to GA.
• Kubernetes Enhancement Proposal: Graduate custom.metrics.k8s.io to stable #5269
• Discussion Link: Graduate custom.metrics.k8s.io API to GA kubernetes#112735
• Primary contact (assignee): @rexagod
• Responsible SIGs: sig-instrumentation
• Enhancement target (which target equals to which milestone):
  • Alpha release target (x.y): 1.6
  • Beta release target (x.y): 1.6
  • Stable release target (x.y):
• Alpha
  • KEP (k/enhancements) update PR(s): N/A
  • Code update PR(s): k/k (PR) & k/metrics mirror (note that this migration was also the first time v1alpha1 appeared).
  • Docs (k/website) update PR(s): N/A
• Beta
  • KEP (k/enhancements) update PR(s): No dedicated KEP, incorporated as a part of Arbitrary/Custom Metrics in the Horizontal Pod Autoscaler #117 & Monitoring Pipeline Metrics HPA API #118.
  • Code (k/k) update PR(s): Graduate custom metrics API to v1beta1 kubernetes#50920
  • Docs (k/website) update(s): Add generated references for metric APIs website#41212
• Stable
  • KEP (k/enhancements) update PR(s): Graduate custom.metrics.k8s.io to stable #5269
  • Code (k/k) update PR(s):
  • Docs (k/website) update(s):

Please note that the ALPHA release pre-dates the KEP process.