[improve][broker] Use atomic counter for ongoing transaction count

[3pacccccc]

Motivation

Currently, txnMetaMap.size() is used to determine if a new transaction can be added and for metrics reporting. However, txnMetaMap is a ConcurrentSkipListMap, and calling size() is an O(n) operation, which can consume significant system resources, especially under high transaction loads.

This PR replaces the use of txnMetaMap.size() with a dedicated atomic counter (onGoingTxnCount) to track the number of ongoing transactions in constant time (O(1)). This improves performance and reduces overhead in transaction management.

Modifications

1. Introduced a LongAdder field onGoingTxnCount in MLTransactionMetadataStore to maintain the ongoing transaction count.
2. Updated all relevant methods to increment/decrement the counter when transactions are added or removed.
3. Refactored test cases to use getOnGoingTxnCount() instead of accessing the map via reflection, improving test clarity and maintainability.
4. Ensured metrics and coordinator stats now use the atomic counter instead of txnMetaMap.size().

Verifying this change

• [x] Make sure that the change passes the CI checks.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• [ ] Dependencies (add or upgrade a dependency)
• [ ] The public API
• [ ] The schema
• [ ] The default values of configurations
• [ ] The threading model
• [ ] The binary protocol
• [ ] The REST endpoints
• [ ] The admin CLI options
• [ ] The metrics
• [ ] Anything that affects deployment

Documentation

• [ ] doc
• [ ] doc-required
• [x] doc-not-needed
• [ ] doc-complete

Matching PR in forked repository

PR in forked repository: https://github.com/3pacccccc/pulsar/pull/34

[codecov-commenter]

Codecov Report

:x: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 74.37%. Comparing base (270120c) to head (2b4c3c8). :warning: Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
...n/coordinator/impl/MLTransactionMetadataStore.java	84.61%	0 Missing and 2 partials :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #25053      +/-   ##
============================================
- Coverage     74.48%   74.37%   -0.11%     
+ Complexity    34186    34116      -70     
============================================
  Files          1921     1921              
  Lines        150376   150394      +18     
  Branches      17469    17472       +3     
============================================
- Hits         112005   111857     -148     
- Misses        29499    29619     +120     
- Partials       8872     8918      +46     

Flag	Coverage Δ
inttests	26.46% <23.07%> (+0.11%)	:arrow_up:
systests	22.87% <0.00%> (-0.05%)	:arrow_down:
unittests	73.89% <84.61%> (-0.12%)	:arrow_down:

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

Files with missing lines	Coverage Δ
...n/coordinator/impl/MLTransactionMetadataStore.java	81.51% <84.61%> (-0.22%)	:arrow_down:

... and 97 files with indirect coverage changes

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