Add info about command metrics for diff-proxy mode

[b1ron]

Description

Closes #3242.

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

Merging #3226 (ce39d91) into main (36ce153) will decrease coverage by 0.14%.
Report is 2 commits behind head on main.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3226      +/-   ##
==========================================
- Coverage   75.76%   75.62%   -0.14%     
==========================================
  Files         396      396              
  Lines       22196    22219      +23     
==========================================
- Hits        16816    16803      -13     
- Misses       4409     4444      +35     
- Partials      971      972       +1     

see 7 files with indirect coverage changes

Flag	Coverage Δ
hana	?
integration	72.07% <ø> (-0.10%)	⬇️
mongodb	5.29% <ø> (-0.01%)	⬇️
pg	65.11% <ø> (-0.10%)	⬇️
shard-1	56.83% <ø> (-0.05%)	⬇️
shard-2	55.47% <ø> (-0.08%)	⬇️
shard-3	56.18% <ø> (-0.05%)	⬇️
sqlite	47.33% <ø> (-0.05%)	⬇️
unit	24.54% <ø> (-0.22%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[AlekSi]

Let's do what #3242 says now

[b1ron]

Closing because work is redirected to #3242.