vtorc: add flag to set collection metric retention/ttl

[timvaillancourt]

Description

This PR adds the flag --discovery-collection-metrics-ttl duration for setting the time-to-live for discovery collection metrics (default 2m0s). This data seems to be used for the aggregated metrics HTTP API only.

This replaces the hardcoded 120 seconds that exists today. In some situations this can lead to a lot of metrics being stored, for example a host monitoring 3000 tablets every 2s will store 180,000 metrics at the 120 second expiry

Related Issue(s)

Resolves #18620

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.49%. Comparing base (b079554) to head (7049825).

Files with missing lines	Patch %	Lines
go/vt/vtorc/config/config.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18619      +/-   ##
==========================================
- Coverage   67.49%   67.49%   -0.01%     
==========================================
  Files        1607     1607              
  Lines      263518   263522       +4     
==========================================
+ Hits       177849   177851       +2     
- Misses      85669    85671       +2     

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[timvaillancourt]

Unrelated fix to my last PR