feat: add audit_last_run_end_time metric

[stek29]

What this PR does / why we need it:
It adds a new metric to track last finishing time for audit runs, to ensure that audit runs do actually finish.
It also updates description of existing metric to note that it only shows time when audit has started, and not when audit has finished.

See #2197 for more info.

[stek29]

I haven't touched versioned website docs – not sure if they should be updated yet.

[codecov-commenter]

Codecov Report

Merging #2235 (4a0bdc2) into master (8f6d95a) will increase coverage by 0.18%.
The diff coverage is 61.90%.

❗ Current head 4a0bdc2 differs from pull request most recent head b5acc5b. Consider uploading reports for the commit b5acc5b to get more accurate results

@@            Coverage Diff             @@
##           master    #2235      +/-   ##
==========================================
+ Coverage   54.34%   54.52%   +0.18%     
==========================================
  Files         111      111              
  Lines        9538     9553      +15     
==========================================
+ Hits         5183     5209      +26     
+ Misses       3954     3948       -6     
+ Partials      401      396       -5     

Flag	Coverage Δ
unittests	54.52% <61.90%> (+0.18%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/audit/manager.go	0.00% <0.00%> (ø)
pkg/audit/stats_reporter.go	76.66% <81.25%> (-0.89%)	⬇️
pkg/readiness/object_tracker.go	83.98% <0.00%> (+1.06%)	⬆️
...onstrainttemplate/constrainttemplate_controller.go	59.71% <0.00%> (+3.59%)	⬆️

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[ritazh]

I haven't touched versioned website docs – not sure if they should be updated yet.

You don't need to update the versioned website docs since this update will only be available after v3.10 is released.

[stek29]

@ritazh it might make sense to update versioned docs to update description of gatekeeper_audit_last_run_time since it’s inaccurate.

[stek29]

@ritazh I've updated description in older versioned docs, but I can remove those changes if you'd prefer me to

[ritazh]

LGTM

[maxsmythe]

LGTM