Add basic logging for PostgreSQL backend

[AlekSi]

Description

Currently, there is none.

Refs #3554.

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 #3560 (82073e4) into main (f748ba9) will decrease coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3560      +/-   ##
==========================================
- Coverage   61.37%   61.24%   -0.14%     
==========================================
  Files         426      426              
  Lines       27192    27200       +8     
==========================================
- Hits        16689    16658      -31     
- Misses       9512     9548      +36     
- Partials      991      994       +3     

Files	Coverage Δ
...ternal/backends/postgresql/metadata/pool/opendb.go	61.25% <100.00%> (+4.30%)	⬆️

... and 6 files with indirect coverage changes

Flag	Coverage Δ
filter-true	57.79% <100.00%> (-0.13%)	⬇️
hana-1	?
integration	57.79% <100.00%> (-0.13%)	⬇️
mongodb-1	4.51% <0.00%> (-0.01%)	⬇️
postgresql-1	41.11% <100.00%> (+0.11%)	⬆️
postgresql-2	40.64% <100.00%> (+0.01%)	⬆️
postgresql-3	39.54% <100.00%> (+0.01%)	⬆️
sort-false	57.79% <100.00%> (-0.13%)	⬇️
sqlite-1	40.99% <0.00%> (-0.02%)	⬇️
sqlite-2	41.17% <0.00%> (-0.02%)	⬇️
sqlite-3	39.45% <0.00%> (-0.02%)	⬇️
unit	22.36% <100.00%> (+0.01%)	⬆️

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