Fix MySQL collection stats

[adetunjii]

Description

Closes #3413

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Milestone (Next), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 59 lines in your changes are missing coverage. Please review.

Project coverage is 73.69%. Comparing base (4a67734) to head (2c232ae).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4145      +/-   ##
==========================================
- Coverage   73.96%   73.69%   -0.28%     
==========================================
  Files         353      353              
  Lines       22680    22738      +58     
==========================================
- Hits        16775    16756      -19     
- Misses       4633     4704      +71     
- Partials     1272     1278       +6     

Files	Coverage Δ
internal/backends/mysql/collection.go	0.00% <0.00%> (ø)

... and 9 files with indirect coverage changes

Flag	Coverage Δ
filter-true	66.97% <0.00%> (-0.41%)	⬇️
integration	66.97% <0.00%> (-0.41%)	⬇️
mongodb-1	4.74% <0.00%> (-0.02%)	⬇️
postgresql-1	48.04% <0.00%> (-0.23%)	⬇️
postgresql-2	?
postgresql-3	49.09% <0.00%> (-0.13%)	⬇️
sqlite-1	47.30% <0.00%> (-0.13%)	⬇️
sqlite-2	47.78% <0.00%> (-0.12%)	⬇️
sqlite-3	?
unit	32.56% <0.00%> (-0.09%)	⬇️

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[chilagrow]

Thanks for your contribution!