Add operators support for $addFields

[noisersup]

Description

Closes #2679.

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 #2850 (f042aa6) into main (ced4756) will increase coverage by 0.10%.
The diff coverage is 84.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2850      +/-   ##
==========================================
+ Coverage   63.37%   63.48%   +0.10%     
==========================================
  Files         451      451              
  Lines       23465    23511      +46     
==========================================
+ Hits        14872    14926      +54     
+ Misses       7628     7619       -9     
- Partials      965      966       +1     

Impacted Files	Coverage Δ
.../common/aggregations/operators/operators_errors.go	100.00% <ø> (ø)
internal/handlers/commonerrors/error.go	80.00% <ø> (ø)
internal/handlers/commonerrors/errorcode_string.go	80.00% <ø> (ø)
...andlers/common/aggregations/operators/operators.go	71.73% <70.58%> (-1.60%)	⬇️
internal/handlers/common/add_fields_iterator.go	85.71% <80.35%> (-14.29%)	⬇️
...nal/handlers/common/aggregations/operators/type.go	79.71% <100.00%> (ø)
...ommon/aggregations/stages/projection/projection.go	87.44% <100.00%> (+2.45%)	⬆️
...al/handlers/common/aggregations/stages/validate.go	71.69% <100.00%> (+12.68%)	⬆️

... and 1 file with indirect coverage changes

Flag	Coverage Δ
integration	56.68% <84.00%> (+0.12%)	⬆️
mongodb	4.15% <0.00%> (-0.01%)	⬇️
pg	56.60% <84.00%> (+0.12%)	⬆️
shard-1	40.84% <12.00%> (-0.03%)	⬇️
shard-2	41.58% <0.00%> (-0.09%)	⬇️
shard-3	45.09% <84.00%> (+0.04%)	⬆️
unit	24.34% <0.00%> (-0.06%)	⬇️

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

[mergify]

@noisersup this pull request has merge conflicts.

[mergify]

@noisersup this pull request has merge conflicts.

[mergify]

@noisersup this pull request has merge conflicts.

[chilagrow]

Will there be another PR? The issue mentions the support of operator for common/projection.go but I'm not seeing the change related to that 🤔

[AlekSi]

At the very least, there is a merge conflict

[chilagrow]

👋 @noisersup, this PR does not address what I analysed on the issue, maybe there is a confusion 🤔 The issue was about supporting $group $project and query projection to support recursive operator. At the time of analysis, $addField and $set were not implemented so that's why only 3 of them support operators. Is there a confusion in the issue where it says Supported stages support operators in the issue please? If we want to handle $addField in this issue, that's great but could you create issues for remaining $group, $project and query projection and $set too please? Maybe you've already created them 🤔

[noisersup]

The issue was about supporting $group $project and query projection to support recursive operator.

I believe that from the description and title it seems that we need to support operators in more stages than only in $project. As we want to cut the scope for this issue I've created another issue that covers all of remaining operators -> #2881

[rumyantseva]

Looks good to me, maybe Chi can spot something as she analyzed the original task, but to me the code looks consistent with the rest of aggregation.

[chilagrow]

Thanks for this!