Fix index name generation

[rumyantseva]

Description

Closes #3375.

Passed:

 task test-integration-postgresql SHARD_INDEX=0 SHARD_TOTAL=0 TEST_RUN='TestAggregateCompatGroup'

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 #3511 (185672a) into main (3502cc0) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3511      +/-   ##
==========================================
+ Coverage   74.84%   74.90%   +0.06%     
==========================================
  Files         426      426              
  Lines       26966    26968       +2     
==========================================
+ Hits        20182    20200      +18     
+ Misses       5609     5566      -43     
- Partials     1175     1202      +27     

Files	Coverage Δ
internal/backends/postgresql/metadata/registry.go	71.47% <100.00%> (+6.37%)	⬆️

... and 25 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	71.78% <100.00%> (+0.16%)	⬆️
hana-1	?
integration	71.78% <100.00%> (+0.11%)	⬆️
mongodb-1	4.53% <0.00%> (-0.01%)	⬇️
pg-1	39.16% <0.00%> (-0.52%)	⬇️
pg-2	42.26% <0.00%> (-0.45%)	⬇️
pg-3	42.63% <0.00%> (-0.64%)	⬇️
postgresql-1	38.84% <100.00%> (+19.96%)	⬆️
postgresql-2	40.85% <100.00%> (+24.98%)	⬆️
postgresql-3	42.44% <100.00%> (+21.78%)	⬆️
sort-false	71.78% <100.00%> (+0.69%)	⬆️
sort-true	?
sqlite-1	38.36% <0.00%> (-0.05%)	⬇️
sqlite-2	41.47% <0.00%> (-0.12%)	⬇️
sqlite-3	42.07% <0.00%> (-0.07%)	⬇️
unit	25.72% <100.00%> (-0.02%)	⬇️

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

[chilagrow]

Thank you!

[rumyantseva]

Not ready yet, tests to be added

[AlekSi]

Please re-request when ready

[noisersup]

LGTM