Implement initial regression detection

[rwstauner]

This sets up regression detection for the ratio_in_yjit metric using the idea of

Over the last 30-60 days, what was the lowest value? If we get a new value that’s 0.5 * stddev below the “floor” twice in a row, flag it?

I included the data that goes into the analysis to help us fine tune the algorithm and to make it easier to decide if a notification is worth investigating.

This is more or less what the slack notification would like:

refs #366

[maximecb]

Nice! Code changes look relatively compact too 👌

It looks for streaks to identify stable values.

Neat! I don't fully understand how the streaks are computed though, is it some average +/- some delta percentage?

Small details wrt the slack notification:

When showing regressions, it would be nice to use the same capitalization formatting as the stat name eg ratio_in_yjit Might also be nice to show a percentage first e.g. -2.1%, from xxx% to yyy% (otherwise you kinda have to mentally calculate the percentage).

[rwstauner]

This shows it would have caught the regression last Sep (#366):

$ bin/analysis --benchmarks=railsbench build/raw-benchmark-data.prod/raw_benchmark_data/x86_64/2024-09
ratio_in_yjit x86_64_yjit_stats
 railsbench streaks: [[99.7, 2], [99.81, 4], [99.7, 6], [99.48, 16]]
            highest_streak_value: 99.81
            longest_streak: [99.48, 16]
            geomean: 99.58991325387635
            regression: dropped 0.33% from 99.81 to 99.48
$ bin/analysis --benchmarks=railsbench build/raw-benchmark-data.prod/raw_benchmark_data/x86_64/2024-10
ratio_in_yjit x86_64_yjit_stats
 railsbench streaks: [[99.48, 13], [99.6, 1], [99.57, 2], [99.6, 14]]
            highest_streak_value: 99.6
            longest_streak: [99.6, 14]
            geomean: 99.54598300131984
$ bin/analysis --benchmarks=railsbench build/raw-benchmark-data.prod/raw_benchmark_data/x86_64/2024-11
ratio_in_yjit x86_64_yjit_stats
 railsbench streaks: [[99.6, 26], [99.82, 4]]
            highest_streak_value: 99.82
            longest_streak: [99.6, 26]
            geomean: 99.62930529510919

and in late November the number goes back up.

[maximecb]

I will let you decide when this should be merged and how it should be tested 👍