Implement MsgDBStats for Tigris

[rumyantseva]

Description

This PR closes #774

Readiness checklist

• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #1047 (2d4c31c) into main (1533c4b) will increase coverage by 23.74%.
The diff coverage is 73.84%.

@@             Coverage Diff             @@
##             main    #1047       +/-   ##
===========================================
+ Coverage   40.12%   63.87%   +23.74%     
===========================================
  Files         240      240               
  Lines       11281    11344       +63     
===========================================
+ Hits         4527     7246     +2719     
+ Misses       6121     3252     -2869     
- Partials      633      846      +213     

Impacted Files	Coverage Δ
internal/handlers/tigris/msg_dbstats.go	74.24% <73.84%> (+74.24%)	⬆️
internal/util/version/version.go	55.00% <0.00%> (-5.00%)	⬇️
internal/bson/document.go	47.09% <0.00%> (+0.68%)	⬆️
internal/clientconn/conn.go	46.38% <0.00%> (+0.76%)	⬆️
integration/helpers.go	83.42% <0.00%> (+1.06%)	⬆️
internal/wire/msg_body.go	38.09% <0.00%> (+1.58%)	⬆️
internal/types/types.go	85.88% <0.00%> (+2.35%)	⬆️
internal/wire/msg_header.go	71.42% <0.00%> (+2.38%)	⬆️
internal/handlers/pg/pgdb/tables.go	64.70% <0.00%> (+2.94%)	⬆️
internal/clientconn/listener.go	79.16% <0.00%> (+3.12%)	⬆️
... and 53 more

Flag	Coverage Δ
integration	59.87% <73.84%> (+29.58%)	⬆️
mongodb	18.23% <ø> (+0.02%)	⬆️
pg	62.64% <ø> (?)
tigris	27.58% <73.84%> (+0.32%)	⬆️
unit	25.15% <0.00%> (-0.11%)	⬇️

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

[w84thesun]

LGTM.

[noisersup]

I don't see any implementation of freeStorage field. Is it in the scope of this task?

[rumyantseva]

@noisersup I don't think we have it implemented for Postgres, and I believe now we need to focus on the main functionality, so freeStorage is for future, not for now.

[noisersup]

LGTM