feat(minor): Enable Swift 6 mode and support specifying units per metric

[hassila]

Description

Swift 6 benchmark targets should now be possible with:

let benchmarks: @Sendable () -> Void = {
...
}

Make it possible to specify output units for the text output for a given metric by specifying it in the configuration.

E.g.

    Benchmark.defaultConfiguration.units = [.peakMemoryResident: .mega, .peakMemoryVirtual: .giga]

Also add the ability to override the time units from the command line using --time-units.

E.g.

swift package benchmark --time-units microseconds

This update also displays overflowing numeric values in scientific notation in the text output:

╒═══════════════════════════════════════════════════════════════════════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╕
│ Test                                                                      │        p0 │       p25 │       p50 │       p75 │       p90 │       p99 │      p100 │   Samples │
╞═══════════════════════════════════════════════════════════════════════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╡
│ Samples:All metrics, full concurrency, async (ns) *                       │  8.08e+07 │  8.34e+07 │  8.38e+07 │  8.40e+07 │  8.41e+07 │  8.46e+07 │  8.49e+07 │       183 │
├───────────────────────────────────────────────────────────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│ Samples:Counter, custom metric thresholds (ns) *                          │      2375 │      2417 │      2417 │      2459 │      2501 │      2709 │      4334 │      4753 │
├───────────────────────────────────────────────────────────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│ Samples:Extended + custom metrics (ns) *                                  │      1875 │      2000 │      2000 │      2041 │      2167 │      2667 │      8750 │       707 │
├───────────────────────────────────────────────────────────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│ Samples:Extended metrics (ns) *                                           │      1750 │      1875 │      1875 │      1917 │      1958 │      2209 │      3250 │       705 │
╘═══════════════════════════════════════════════════════════════════════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╛

Also updates documentation.

Addresses:
#306
and
#237
#293
#277
#258

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

Minimal checklist:

• I have performed a self-review of my own code
• I have added DocC code-level documentation for any public interfaces exported by the package
• I have added unit and/or integration tests that prove my fix is effective or that my feature works

[github-actions]

Pull request had an unknown failure
Pull request benchmark comparison [ubuntu-latest] with 'main' run at 2025-02-11T12:52:06+00:00

[codecov]

Codecov Report

Attention: Patch coverage is 26.26263% with 73 lines in your changes missing coverage. Please review.

Project coverage is 70.21%. Comparing base (51fffb6) to head (68fe709).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/Benchmark/BenchmarkResult.swift	11.76%	60 Missing ⚠️
Sources/Benchmark/BenchmarkExecutor.swift	42.86%	8 Missing ⚠️
Sources/Benchmark/Benchmark.swift	54.55%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #309      +/-   ##
==========================================
- Coverage   71.41%   70.21%   -1.20%     
==========================================
  Files          32       32              
  Lines        3791     3867      +76     
==========================================
+ Hits         2707     2715       +8     
- Misses       1084     1152      +68     

Files with missing lines	Coverage Δ
Sources/Benchmark/BenchmarkMetric.swift	62.57% <ø> (ø)
Sources/Benchmark/BenchmarkRunner.swift	60.22% <100.00%> (+1.13%)	⬆️
Sources/Benchmark/Statistics.swift	66.92% <ø> (ø)
Tests/BenchmarkTests/BenchmarkResultTests.swift	100.00% <ø> (ø)
Tests/BenchmarkTests/BenchmarkRunnerTests.swift	92.31% <100.00%> (+0.20%)	⬆️
Sources/Benchmark/Benchmark.swift	69.71% <54.55%> (-1.02%)	⬇️
Sources/Benchmark/BenchmarkExecutor.swift	91.16% <42.86%> (-1.31%)	⬇️
Sources/Benchmark/BenchmarkResult.swift	61.93% <11.76%> (-8.20%)	⬇️

Files with missing lines	Coverage Δ
Sources/Benchmark/BenchmarkMetric.swift	62.57% <ø> (ø)
Sources/Benchmark/BenchmarkRunner.swift	60.22% <100.00%> (+1.13%)	⬆️
Sources/Benchmark/Statistics.swift	66.92% <ø> (ø)
Tests/BenchmarkTests/BenchmarkResultTests.swift	100.00% <ø> (ø)
Tests/BenchmarkTests/BenchmarkRunnerTests.swift	92.31% <100.00%> (+0.20%)	⬆️
Sources/Benchmark/Benchmark.swift	69.71% <54.55%> (-1.02%)	⬇️
Sources/Benchmark/BenchmarkExecutor.swift	91.16% <42.86%> (-1.31%)	⬇️
Sources/Benchmark/BenchmarkResult.swift	61.93% <11.76%> (-8.20%)	⬇️

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