Investigate counters in reverse replication job

[aasthabharill]

https://b.corp.google.com/issues/470738963

[gemini-code-assist]

Summary of Changes

Hello @aasthabharill, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a persistent flakiness issue in a critical Spanner to MySQL replication test by refining the core logic for processing and comparing record sequences. The changes enhance the robustness of the data pipeline by accurately identifying record states and introducing explicit logging for duplicate entries, which is crucial for maintaining data integrity and debugging in high-throughput, custom transformation environments.

Highlights

• Flaky Test Fix: This pull request addresses and fixes a flakiness issue identified in the SpannerToMySqlCustomTransformationLT.reverseReplication1KTpsWithCustomTransformation test.
• Record Sequence Comparison Logic: The comparison logic for record sequence numbers within SourceWriterFn.java has been updated from a strict greater than (>) to a greater than or equal to (>=) when determining if a shadow table record is considered 'ahead' of a Spanner record. This change ensures more accurate handling of record ordering and potential re-processing scenarios.
• Duplicate Record Detection and Logging: New logging has been introduced in SourceWriterFn.java to explicitly detect and report duplicate records. If a shadow table record and a Spanner record share the exact same processed commit timestamp and record sequence, a log message will now indicate that a duplicate record has been found.

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[codecov]

Codecov Report

❌ Patch coverage is 36.36364% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.70%. Comparing base (08838cc) to head (684b7d9).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...leport/v2/templates/transforms/SourceWriterFn.java	36.36%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3134      +/-   ##
============================================
+ Coverage     50.66%   50.70%   +0.03%     
- Complexity     5051     5072      +21     
============================================
  Files           974      974              
  Lines         59828    59882      +54     
  Branches       6527     6539      +12     
============================================
+ Hits          30314    30363      +49     
- Misses        27370    27375       +5     
  Partials       2144     2144              

Components	Coverage Δ
spanner-templates	70.83% <36.36%> (+0.03%)	⬆️
spanner-import-export	68.97% <ø> (ø)
spanner-live-forward-migration	80.06% <ø> (+0.01%)	⬆️
spanner-live-reverse-replication	77.64% <36.36%> (-0.09%)	⬇️
spanner-bulk-migration	88.35% <ø> (+0.06%)	⬆️

Files with missing lines	Coverage Δ
...leport/v2/templates/transforms/SourceWriterFn.java	81.76% <36.36%> (-2.81%)	⬇️

... and 12 files with indirect coverage changes

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