Create generic statsReporter in metrics package

[Leo6Leo]

Proposed Changes

• Create the generic StatsReporter in the metrics package
• Merge all the existing statsReporter to use this generic statsreporter
• Modify the corresponding unit tests

Pre-review Checklist

• At least 80% unit test coverage
• E2E tests for any new behavior
• Docs PR for any user-facing impact
• Spec PR for any new API feature
• Conformance test for any change to the spec

Release Note

Docs

[knative-prow]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[Cali0707]

@Leo6Leo can you rebase?

[knative-prow]

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[Leo6Leo]

/cc @creydr

I re-write the statsReport to make it more generic and moved it to the metrics package by following the suggestions you left here.

Please let me know if this is on the right track, then I will proceed changing the statsReporter for channel, broker ingress... to use the new generic statsReporter.

[creydr]

@Leo6Leo can you check on the failing tests?

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.74%. Comparing base (71d5d5f) to head (e1aa68a).
Report is 1 commits behind head on main.

Files	Patch %	Lines
pkg/metrics/stats_reporter.go	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8084      +/-   ##
==========================================
+ Coverage   67.71%   67.74%   +0.03%     
==========================================
  Files         370      371       +1     
  Lines       17946    17965      +19     
==========================================
+ Hits        12152    12171      +19     
  Misses       5020     5020              
  Partials      774      774              

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