fix(flink): remove pre-seed in metrics in GlobalRecordLevelIndexBackend and RecordLevelIndexBackend

[skywalker0618]

Describe the issue this Pull Request addresses

The pre-seed logic in metrics in GlobalRecordLevelIndexBackend and RecordLevelIndexBackend are not needed by production code and they only existed for test purpose. This PR removes this unnecessary step and fix all test cases.

Fixes #18734

Summary and Changelog

Remove pre-seed in metrics in GlobalRecordLevelIndexBackend and RecordLevelIndexBackend and fix test cases

Impact

None

Risk Level

None. Production code doesn't need pre-seed.

Documentation Update

N/A

Contributor's checklist

• Read through contributor's guide
• Enough context is provided in the sections above
• Adequate tests were added if applicable

[hudi-agent]

🤖 This review was generated by an AI agent and may contain mistakes. Please verify any suggestions before applying.

Thanks for the cleanup! This PR removes the UnregisteredMetricsGroup pre-seed from both index backend constructors and updates tests to register metrics explicitly when they exercise the metric-touching paths. I traced the production callers (BucketAssignFunction.initializeState and DynamicBucketAssignFunction.initializeState) and confirmed both invoke registerMetrics(...) immediately after construction, before any get/update/bootstrapPartition call that would dereference metrics, so removing the pre-seed is safe for production paths. No issues flagged from this automated pass — a Hudi committer or PMC member can take it from here for a final review.

cc @yihua

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.81%. Comparing base (ff72186) to head (60c93b0).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #18875   +/-   ##
=========================================
  Coverage     68.81%   68.81%           
- Complexity    29142    29144    +2     
=========================================
  Files          2515     2515           
  Lines        139931   139929    -2     
  Branches      17187    17187           
=========================================
+ Hits          96288    96296    +8     
+ Misses        35864    35854   -10     
  Partials       7779     7779           

Flag	Coverage Δ
common-and-other-modules	44.32% <ø> (-0.01%)	⬇️
hadoop-mr-java-client	44.90% <ø> (-0.01%)	⬇️
spark-client-hadoop-common	48.22% <ø> (-0.01%)	⬇️
spark-java-tests	49.37% <ø> (-0.03%)	⬇️
spark-scala-tests	45.27% <ø> (-0.01%)	⬇️
utilities	37.46% <ø> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...rtitioner/index/GlobalRecordLevelIndexBackend.java	90.16% <ø> (-0.16%)	⬇️
...ink/partitioner/index/RecordLevelIndexBackend.java	56.57% <ø> (-0.29%)	⬇️

... and 12 files with indirect coverage changes

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[hudi-bot]

CI report:

• 60c93b0 Azure: SUCCESS

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