KEP-4346: client-go: Add informer metrics (kubernetes#4349)

* Add informer metrics

* use feature gate

add test describe

fill section: Feature Enablement and Rollback

mitigate memory usage

add memory usage metrics

Signed-off-by: chenk008 <[email protected]>

* add description about prometheus client

* make update-toc

* Update keps/sig-api-machinery/4346-informer-metrics/README.md

Co-authored-by: Han Kang <[email protected]>

* Update keps/sig-api-machinery/4346-informer-metrics/README.md

Co-authored-by: Han Kang <[email protected]>

---------

Signed-off-by: chenk008 <[email protected]>
Co-authored-by: Han Kang <[email protected]>

Author: chenk008