feat: add admission count metric ingress_controller_admission_count

[pmalek]

What this PR does / why we need it:

Which issue this PR fixes

Fixes #

Special notes for your reviewer:

Originally submitted by https://github.com/dweber019 in Kong/kubernetes-ingress-controller#6087.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect significant changes

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[pmalek]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.You have signed the CLA already but the status is still pending? Let us recheck it.

@dweber019 Please sign the CLA here if you're still interested to get the contribution here.