[Release 21.0] [Direct PR] Conclude txn in VTAdmin and GetUnresolvedTransactions bug fix

[GuptaManan100]

Description

This PR does 2 things -

1. It backports VTAdmin: Support for conclude txn and abandon age param #16834 to release-21 since it has been deemed to be a feature that is required for atomic transactions.
2. It fixes the bug in GetUnresolvedTransactions wherein we don't show the complete list of participants, but omit the metadata manager shard.

I will forward port the bug fix against main. This PR is done directly so that it makes it into the RC planned tomorrow.

Related Issue(s)

• RFC: Atomic Distributed Transactions #16245

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 16.66667% with 35 lines in your changes missing coverage. Please review.

Project coverage is 69.43%. Comparing base (34df88f) to head (c2473e2).
Report is 2 commits behind head on release-21.0.

Files with missing lines	Patch %	Lines
go/vt/vtadmin/api.go	6.66%	14 Missing ⚠️
go/vt/vtadmin/http/transactions.go	0.00%	12 Missing ⚠️
go/vt/vtadmin/http/request.go	0.00%	9 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           release-21.0   #16949   +/-   ##
=============================================
  Coverage         69.42%   69.43%           
=============================================
  Files              1570     1570           
  Lines            203814   203841   +27     
=============================================
+ Hits             141506   141529   +23     
- Misses            62308    62312    +4     

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