Add uniform block tracking stats

[joshkang97]

Summary

Add tracking of uniform index blocks as a table property (num_uniform_blocks). When uniform_cv_threshold is set, the block builder detects uniformly distributed keys via coefficient of variation of key gaps. This information is now surfaced end-to-end: from block building through index builders to SST table properties.

Key changes

• BlockBuilder: Persist the uniformity result from ScanForUniformity() as a member (is_uniform_) exposed via IsUniform(), rather than a local variable discarded after Finish().
• Index builders: All three index builder types (ShortenedIndexBuilder, HashIndexBuilder, PartitionedIndexBuilder) implement NumUniformIndexBlocks(). For partitioned indexes, the count accumulates across partition Finish() calls.
• Table property serialization: Added TablePropertiesNames::kNumUniformBlocks ("rocksdb.num.uniform.blocks") with full serialization/deserialization support. Without this, the property was computed in memory but never persisted to SST files.
• Table property aggregation: num_uniform_blocks included in Add() and GetAggregatablePropertiesAsMap() for correct cross-SST aggregation.

Test Plan

• Unit tests: block_test (7504 passed), table_test (6917 passed), table_properties_collector_test (8 passed).
• End-to-end verification via db_bench + sst_dump:
  • Positive: ./db_bench --benchmarks=fillseq,compact --num=10000 --uniform_cv_threshold=0.2 --index_shortening_mode=0 --compression_type=none produces SST with # uniform blocks: 1.
  • Negative: Same with --uniform_cv_threshold=-1 (disabled) produces # uniform blocks: 0.

[meta-codesync]

@joshkang97 has imported this pull request. If you are a Meta employee, you can view this in D98343170.

[github-actions]

⚠️ clang-tidy: 1 warning(s) on changed lines

Completed in 126.3s.

Summary by check

Check	Count
cert-err58-cpp	1
Total	1

Details

table/table_properties.cc (1 warning(s))

table/table_properties.cc:287:41: warning: initialization of 'kNumUniformBlocks' with static storage duration may throw an exception that cannot be caught [cert-err58-cpp]