Add TransactionsProcessed metric to track transactions at VTGate

[harshit-gangal]

Description

This PR introduces a new counter metric TransactionsProcessed to VTGate to track the number of transactions processed, categorized by:

• Shard distribution: single or cross
• Transaction type: read only or read write

Related Issue(s)

• Closes RFC: New vtgate Metrics #17585

Checklist

• 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

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[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 67.54%. Comparing base (9e734f8) to head (7d4cfd7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/tx_conn.go	87.50%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18171   +/-   ##
=======================================
  Coverage   67.53%   67.54%           
=======================================
  Files        1601     1601           
  Lines      261484   261509   +25     
=======================================
+ Hits       176592   176628   +36     
+ Misses      84892    84881   -11     

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