✨ Add a metrics to expose informer cache's length

[halfcrazy]

Fix #3182

metrics explain

# HELP controller_runtime_cache_resources Number of resources cached in the controller-runtime local cache, broken down by group, version, kind
# TYPE controller_runtime_cache_resources gauge
controller_runtime_cache_resources{group="webapp.my.domain",kind="Guestbook",version="v1"} 1

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: halfcrazy
Once this PR has been reviewed and has the lgtm label, please assign vincepri for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[k8s-ci-robot]

Hi @halfcrazy. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[xigang]

@halfcrazy Informer metrics have already been implemented in the Kubernetes repo and will be synced to client-go. There's no need to implement them within controller-runtime.

xref: kubernetes/kubernetes#129160

[sbueringer]

@xigang thx for chiming in!