Use iterator in aggregation stages

[chilagrow]

Description

Closes #1889.

• $group needs to accumulate all values in the memory so it reads all document.
• $unwind is a bit tricky to handle array iterator, it reads all document.

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• 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), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #2480 (734151c) into main (ce28270) will decrease coverage by 0.33%.
The diff coverage is 66.98%.

❗ Current head 734151c differs from pull request most recent head 66cb98d. Consider uploading reports for the commit 66cb98d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2480      +/-   ##
==========================================
- Coverage   65.10%   64.78%   -0.33%     
==========================================
  Files         399      400       +1     
  Lines       19747    19769      +22     
==========================================
- Hits        12857    12808      -49     
- Misses       5968     6043      +75     
+ Partials      922      918       -4     

Impacted Files	Coverage Δ
...rnal/handlers/common/aggregations/stages/stages.go	20.00% <ø> (ø)
internal/handlers/common/limit_iterator.go	82.75% <0.00%> (-2.96%)	⬇️
internal/handlers/tigris/msg_aggregate.go	0.00% <0.00%> (ø)
...l/handlers/common/aggregations/stages/collstats.go	83.92% <25.00%> (-12.15%)	⬇️
...ernal/handlers/common/aggregations/stages/group.go	87.71% <57.14%> (-1.51%)	⬇️
...rnal/handlers/common/aggregations/stages/unwind.go	82.47% <62.50%> (-2.31%)	⬇️
internal/handlers/pg/msg_aggregate.go	83.18% <77.77%> (+1.78%)	⬆️
internal/handlers/common/count_iterator.go	91.17% <91.17%> (ø)
...ernal/handlers/common/aggregations/stages/count.go	100.00% <100.00%> (ø)
...ernal/handlers/common/aggregations/stages/limit.go	81.25% <100.00%> (+11.25%)	⬆️
... and 4 more

... and 4 files with indirect coverage changes

Flag	Coverage Δ
integration	58.00% <66.98%> (+<0.01%)	⬆️
mongodb	5.11% <0.00%> (-0.01%)	⬇️
pg	57.92% <66.98%> (+<0.01%)	⬆️
unit	25.28% <0.00%> (-0.39%)	⬇️

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

[mergify]

@chilagrow this pull request has merge conflicts.

[rumyantseva]

I like the implementation (it looks similar to how FilterIterator works), but I see quite a lot of things fail, so this PR needs some changes to make the linters and tests pass etc.

[chilagrow]

Thanks @w84thesun for fixing the linter and merging main 🤗

[chilagrow]

I like the implementation (it looks similar to how FilterIterator works), but I see quite a lot of things fail, so this PR needs some changes to make the linters and tests pass etc.

@rumyantseva thanks for noticing this, I think linter/failing tests were due to not being up to date with main branch and few conflicting change. Should be fine now, thanks to @w84thesun 🙏

[w84thesun]

👍

[rumyantseva]

LGTM

[noisersup]

LGTM