[SDK] Fix off-by-one error in Base2ExponentialHistogramAggregation::Merge() downscaling

[martani]

Issue

The count invariant (count == zero_count + sum(bucket_counts)) was being violated during Base2ExponentialHistogramAggregation::Merge() operations when the combined bucket range of two histograms exceeded max_buckets by exactly 1.

Changes

The offending condition if (pos_max_index > pos_min_index + max_buckets) ... fails when the combined range exceeds max_buckets by exactly 1.

Step-by-step example with max_buckets=5 at scale 0:

• Add values 2,4,8,16,32: histogram spans indices 0-4
• Add values 4,8,16,32,64: histogram spans indices 1-5 (64 extends one position beyond the previous range)

During Merge:

• pos_min_index = min(0, 1) = 0
• pos_max_index = max(4, 5) = 5
• combined range = indices 0-5 = 6 positions = max_buckets + 1

Old condition check:

• pos_max_index > pos_min_index + max_buckets
• 5 > 0 + 5
• 5 > 5 = FALSE <-- downscaling NOT triggered (BUG!)

Fixed condition check:

• pos_max_index >= pos_min_index + max_buckets
• 5 >= 0 + 5
• 5 >= 5 = TRUE <-- downscaling correctly triggered

When downscaling wasn't triggered but the range exceeded max_buckets, MergeBuckets() created a result that couldn't fit in the circular buffer, causing bucket counts to be silently lost.

Prometheus rejects histograms with mismatching counts here: https://github.com/prometheus/prometheus/blob/main/model/histogram/histogram.go#L474

For significant contributions please make sure you have completed the following items:

• [ ] CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• [ ] Changes in public API reviewed

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: martani / name: martani (9fbc1d4)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.93%. Comparing base (5fc4707) to head (9fbc1d4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3793   +/-   ##
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  Coverage   89.93%   89.93%           
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  Files         225      225           
  Lines        7163     7163           
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  Hits         6441     6441           
  Misses        722      722           

Files with missing lines	Coverage Δ
...egation/base2_exponential_histogram_aggregation.cc	91.33% <100.00%> (ø)

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[marcalff]

LGTM, with detailed root cause analysis, comments and test cases: great work.

Merci pour le patch.