Function next_benchmark_run_directory 

pub fn next_benchmark_run_directory(
    runs_root: impl AsRef<Path>,
    timestamp: &str,
    git_commit_hash: Option<&str>,
) -> Result<BenchmarkRunDirectory>

Compute the next available benchmark run path for one timestamp and commit.

This function does not create or reserve the returned directory. Callers allocating the same prefix concurrently must synchronize that shared resource or provide unique timestamps.