Feature: add total read size counter

[harryscholes]

I found that I want to know the total number of bytes read when querying a file, not just the distribution of individual IOs. Implemented as a counter metric named vortex.io.read.total_size.

[lwwmanning]

thanks for the contribution @harryscholes !

to fix the DCO check, I recommend running something like git rebase -i origin/develop --signoff (need to signoff every git commit in the PR, see CONTRIBUTING.md for details)

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.59%. Comparing base (89b236b) to head (bd85a8d).
⚠️ Report is 3 commits behind head on develop.

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[onursatici]

Thanks for the contribution, yea sizes would have the reads histogram, all histograms have a count but not a sum.