Adding webhook to prom metrics

[bfoley13]

Description

This changes adds mutation/validation webhooks into our current Prometheus metrics.

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration. Is it a breaking change which will impact consuming tool(s)?

• [x] Unit Tests

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules

[coveralls]

Pull Request Test Coverage Report for Build 6749516416

Warning: This coverage report may be inaccurate.

We've detected an issue with your CI configuration that might affect the accuracy of this pull request's coverage report. To ensure accuracy in future PRs, please see these guidelines. A quick fix for this PR: rebase it; your next report should be accurate.

• 22 of 27 (81.48%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 78.548%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/webhook/nginxingress.go	22	27	81.48%
<!--	Total:	22	27	81.48%	-->

Totals
Change from base Build 6740427943:	0.02%
Covered Lines:	2369
Relevant Lines:	3016

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💛 - Coveralls