Increase default repetitions during codspeed walltime runs, and fix walltime stats computation

[GuillaumeLagrange]

Followup of #33, but the assumption made on what getAdujstedRealTime was doing were wrong

We're also changing the default number of repetitions (round), since we believe 1 is not a good default, as the resulting box plot only has a single element by default

[codspeed-hq]

CodSpeed Performance Report

Merging #34 will not alter performance

_{Comparing cod-1639-fix-codspeed-cpp-walltimetotaltime-and-maybe (da312ec) with main (8718448)¹}

Summary

✅ 196 untouched

Footnotes

1. No successful run was found on main (da312ec) during the generation of this report, so 8718448 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[not-matthias]

Looks good overall. Can we add some tests to ensure the logic works this time?

[Copilot]

Pull Request Overview

This PR refactors the walltime benchmark data collection and statistics computation to support variable iterations per round. Instead of storing pre-aggregated statistics (mean, median, stdev) from Google Benchmark, it now collects raw per-round data (iterations and total time) and computes statistics based on per-iteration times.

Key changes:

• Changed RawWalltimeBenchmark structure to store vectors of iterations and times per round instead of pre-computed aggregates
• Modified statistics computation to calculate per-iteration times and derive all statistics from these values
• Increased default benchmark repetitions to 5 when CODSPEED_WALLTIME is enabled for better statistical reliability

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
core/include/codspeed.h	Updated RawWalltimeBenchmark struct to store vectors of per-round iterations and times instead of pre-aggregated statistics
core/src/walltime.cpp	Refactored statistics computation to work with raw per-round data, computing per-iteration statistics; added constructor to BenchmarkStats and made helper functions static
google_benchmark/src/benchmark.cc	Updated data collection to store per-round iterations and times; changed default repetitions to 5 for walltime mode; minor formatting and comment improvements
.github/workflows/ci.yml	Updated job step name to better reflect its purpose

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[adriencaccia]

We can disregard copilot's comments, since we are asserting the length of the array