feat: Add opt-in cross-thread statistics for multithreaded counters

[shreejaykurhade]

Summary

This PR addresses #1902 by enabling visibility into cross-thread imbalance in multithreaded counters.

Currently, only aggregated (summed) counter values are reported, which makes it difficult to assess fairness across benchmark-created threads. This change introduces an opt-in mechanism to compute and report distribution statistics across threads.

When enabled, benchmark-created thread counter values are retained long enough to compute the benchmark’s configured statistics across threads. The existing summed counter row remains unchanged, and additional aggregate rows:

• _thread_mean
• _thread_median
• _thread_stddev
• _thread_cv

For example, if three benchmark threads report work values 1, 2, and 3, the normal row still reports work=6, while the opt-in thread-stat rows report mean 2, median 2, stddev 1, and cv 50%.

Testing

• cmake --build build
• cmake --build build --config Debug
• ctest --test-dir build --output-on-failure

All 74 CTest tests pass.

[shreejaykurhade]

@dmah42 please review

[dmah42]

is anyone asking for this? i'm beginning to think you've pointed an agent at the codebase and asked it to come up with "improvements".

[shreejaykurhade]

Yes, this is intended to address #1902, which asks for cross-thread statistics to inspect fairness/imbalance in multithreaded counters.

I do use Codex to understand parts of the codebase (e.g., structure and JSON fields), but the design and implementation here are deliberate.

The change is opt-in and doesn’t affect existing behavior.

Happy to adjust if needed.

[shreejaykurhade]

@dmah42 check it please