FEAT: LBC metric collector

[zac-nixon]

Issue

This metric is helpful to identify slowness between registering pods and them going active within the load balancer. Recently, NLB has released faster target registration times and using this prom metric will help validate how fast targets are going active now.

#1834

Description

Refactored the metrics code into it's own package. Introduce an LBC specific Prometheus metric collector that we can expand to collect other interesting metrics.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• [ X Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: zac-nixon
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[k8s-ci-robot]

Hi @zac-nixon. 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.

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[shraddhabang]

/ok-to-test

[shraddhabang]

/lgtm
/approved

[wweiwei-li]

/lgtm
/approved