WIP: irmin-pack.unix: Measure IO activity.

[adatario]

This adds counters that meassure i/o activity at file level.

[metanivek]

I just remembered that the stats here are used as a quick and dirty way to aggregate IO activity for the benchmark summary.

We need to double check the "official" summary printer but you can see how it is used for the bench/irmin-pack tree replay.

irmin/bench/irmin-pack/trace_stat_summary_pp.ml

Lines 384 to 390 in 1f046dd

	pb ~f:(`RM, `RM) "Disk bytes read" (fun s -> s.index.bytes_read);
	pb ~f:(`RM, `RM) "Disk bytes written" (fun s -> s.index.bytes_written);
	pb ~f:(`RM, `RM) "Disk bytes both" (fun s -> s.index.bytes_both);
	`Spacer;
	pb "Disk reads" (fun s -> s.index.nb_reads);
	pb "Disk writes" (fun s -> s.index.nb_writes);
	pb "Disk both" (fun s -> s.index.nb_both);

I am definitely in favor of cleaning this up and making it more clear that these are not "index" stats. Perhaps your new Io stats can help the summary aggregate the index activity as well.

[codecov-commenter]

Codecov Report

Merging #2250 (945791d) into main (1f046dd) will decrease coverage by 0.07%.
The diff coverage is 71.42%.

❗ Current head 945791d differs from pull request most recent head 0557349. Consider uploading reports for the commit 0557349 to get more accurate results

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@@            Coverage Diff             @@
##             main    #2250      +/-   ##
==========================================
- Coverage   68.15%   68.09%   -0.07%     
==========================================
  Files         137      137              
  Lines       16669    16692      +23     
==========================================
+ Hits        11361    11366       +5     
- Misses       5308     5326      +18     

Impacted Files	Coverage Δ
src/irmin-pack/unix/stats_intf.ml	85.00% <ø> (ø)
src/irmin-pack/unix/io.ml	66.10% <71.42%> (-0.28%)	⬇️
src/irmin-pack/unix/stats.ml	56.60% <71.42%> (+3.66%)	⬆️

... and 3 files with indirect coverage changes

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