vtorc: add stats for discovery workers

[timvaillancourt]

Description

This PR adds two gauges to VTOrc:

• DiscoveryWorkers/vtorc_discovery_workers - the number of discovery worker goroutines
• DiscoveryWorkersActive/vtorc_discovery_workers_active - the number of discovery worker goroutines currently doing work

This will be useful for tuning discovery worker/queue concurrency

Related Issue(s)

#17330

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• 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]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 9.09091% with 10 lines in your changes missing coverage. Please review.

Project coverage is 67.55%. Comparing base (526665e) to head (1f5cbac).
Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtorc/logic/vtorc.go	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17937      +/-   ##
==========================================
- Coverage   67.56%   67.55%   -0.02%     
==========================================
  Files        1597     1597              
  Lines      259780   259787       +7     
==========================================
- Hits       175516   175488      -28     
- Misses      84264    84299      +35     

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