vtorc: update metrics in e2e test TestAPIEndpoints

[timvaillancourt]

Description

This PR updates the metrics checked in the TestAPIEndpoints e2e test for VTOrc

Also, a metric I recently added in v22 was renamed from DiscoverInstanceTimings -> DiscoveryInstanceTimings for consistency. The later will be deleted in v24+

Related Issue(s)

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]

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.51%. Comparing base (aa68b96) to head (db69aae).
⚠️ Report is 72 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtorc/logic/vtorc.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #18406    +/-   ##
========================================
  Coverage   67.50%   67.51%            
========================================
  Files        1603     1607     +4     
  Lines      262496   263343   +847     
========================================
+ Hits       177198   177787   +589     
- Misses      85298    85556   +258     

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