Reduce LongSumAggregator.doRecordLong visibility to protected

[thswlsqls]

Fixes #8506

Description

• Narrow LongSumAggregator.Handle.doRecordLong(long) from public to protected.
• The base hook AggregatorHandle.doRecordLong(long) is protected, and all sibling overrides in the internal.aggregator package are protected (DoubleSum, DoubleLastValue, LongLastValue, DoubleExplicitBucketHistogram, DoubleBase2ExponentialHistogram, DropAggregator). LongSumAggregator was the lone public outlier.
• The single-arg doRecordLong(value) is only called by AggregatorHandle.recordLong in the same class; no external caller relies on public.
• Completes the visibility cleanup from Reduce public aggregator API to match latest specification. #3557, which missed this method.
• internal.aggregator is an internal package, so this is not a public API change: no apidiff diff, no CHANGELOG entry.

Testing done

• Visibility-only, no behavior change, test-exempt; no new test added.
• ./gradlew :sdk:metrics:check — 694 tests passed (jApiCmp clean, no apidiff change).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (824334c) to head (a45f51a).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8507      +/-   ##
============================================
- Coverage     78.77%   78.77%   -0.01%     
  Complexity     8579     8579              
============================================
  Files          1009     1009              
  Lines         28993    28993              
  Branches       3599     3599              
============================================
- Hits          22839    22838       -1     
- Misses         5311     5312       +1     
  Partials        843      843              

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