feat: add global counters for bytes_read & iops for benchmarking utility

[westonpace]

This is a small change and not essential but has been useful for me for benchmarking and it might be nice to have it available.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 10 lines in your changes missing coverage. Please review.

Project coverage is 79.02%. Comparing base (38a0a92) to head (b3fa19c).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/lance-io/src/scheduler.rs	25.00%	6 Missing ⚠️
rust/lance-core/src/datatypes.rs	66.66%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3321      +/-   ##
==========================================
+ Coverage   78.97%   79.02%   +0.05%     
==========================================
  Files         246      246              
  Lines       87628    87643      +15     
  Branches    87628    87643      +15     
==========================================
+ Hits        69202    69262      +60     
+ Misses      15558    15513      -45     
  Partials     2868     2868              

Flag	Coverage Δ
unittests	79.02% <50.00%> (+0.05%)	⬆️

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