Test getMore integration test using one connection pool

[chilagrow]

Description

Closes #1807.

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 #2878 (676af8a) into main (2d8f797) will increase coverage by 5.17%.
The diff coverage is 93.75%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2878      +/-   ##
==========================================
+ Coverage   58.86%   64.04%   +5.17%     
==========================================
  Files         451      451              
  Lines       23847    23861      +14     
==========================================
+ Hits        14038    15282    +1244     
+ Misses       8816     7616    -1200     
+ Partials      993      963      -30     

Impacted Files	Coverage Δ
integration/setup/client.go	70.96% <ø> (-10.29%)	⬇️
integration/setup/listener.go	67.97% <88.88%> (+6.23%)	⬆️
ferretdb/ferretdb.go	83.33% <100.00%> (ø)
integration/setup/setup.go	72.02% <100.00%> (+2.24%)	⬆️
integration/setup/setup_compat.go	75.73% <100.00%> (+0.17%)	⬆️
integration/setup/test_helpers.go	93.02% <100.00%> (+1.59%)	⬆️
internal/util/teststress/stress.go	100.00% <100.00%> (ø)

... and 43 files with indirect coverage changes

Flag	Coverage Δ
integration	57.44% <90.00%> (+5.67%)	⬆️
mongodb	4.55% <27.50%> (+0.21%)	⬆️
pg	57.37% <90.00%> (+5.81%)	⬆️
shard-1	42.29% <83.75%> (+0.31%)	⬆️
shard-2	45.44% <76.25%> (+1.35%)	⬆️
shard-3	42.21% <90.00%> (?)
unit	24.13% <100.00%> (-0.01%)	⬇️

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

[AlekSi]

Do we want to test this only for FerretDB?

First of all, we want to understand how MongoDB works. We can't do that if we don't run tests for it.

As far as I know, MongoDB does not provide information related to uniquely identifying a connection for commands.

That's why we fix *mongo.Client's pool size to 1 – it is easy to uniquely identify a connection if there is only one of them.

create a separate client like the first one, ensuring that different clients use different connections.

Also this can be implemented for FerretDB, but I didn't find equivalent check for MongoDB.

Not sure I understand why we can't create a separate *mongo.Client instance for MongoDB. See TestCommandsDiagnosticWhatsMyURI for example.

If we want to ensure FerretDB does use the same connection,

It will use the same connection if there is only one. And if there is only one, if we configure *mongo.Client to use the only one.

[noisersup]

Looks great!

[noisersup]

👍