Do not ConsumeValues in the $group aggregation stage

[adetunjii]

Closes #2863.

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 #3344 (506f278) into main (ee5304e) will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main    #3344      +/-   ##
==========================================
- Coverage   73.94%   73.94%   -0.01%     
==========================================
  Files         413      413              
  Lines       25096    25097       +1     
==========================================
  Hits        18557    18557              
- Misses       5455     5456       +1     
  Partials     1084     1084              

Files Changed	Coverage
...ernal/handlers/common/aggregations/stages/group.go	66.66%

Flag	Coverage Δ
hana-1	?
hana-2	?
hana-3	?
integration	70.95% <66.66%> (-0.01%)	⬇️
mongodb-1	4.76% <0.00%> (-0.01%)	⬇️
pg-1	44.93% <66.66%> (+0.10%)	⬆️
pg-2	45.11% <66.66%> (+<0.01%)	⬆️
pg-3	42.67% <66.66%> (+<0.01%)	⬆️
sqlite-1	43.52% <66.66%> (+<0.01%)	⬆️
sqlite-2	43.30% <66.66%> (+<0.01%)	⬆️
sqlite-3	41.23% <66.66%> (-0.06%)	⬇️
unit	23.23% <0.00%> (-0.04%)	⬇️

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[chilagrow]

Looks great, thanks for your contribution!