Implement setFreeMonitoring stub

[noisersup]

Closes #744.

Checklist

• Tests are added for new functionality or fixed bugs.
• task all passes.

See CONTRIBUTING.md for more details.

[codecov]

Codecov Report

Merging #759 (07ff3e8) into main (9e28a0a) will increase coverage by 0.04%.
The diff coverage is 67.74%.

@@            Coverage Diff             @@
##             main     #759      +/-   ##
==========================================
+ Coverage   65.31%   65.35%   +0.04%     
==========================================
  Files         207      211       +4     
  Lines        8447     8478      +31     
==========================================
+ Hits         5517     5541      +24     
- Misses       2287     2293       +6     
- Partials      643      644       +1     

Impacted Files	Coverage Δ
internal/handlers/common/error.go	82.65% <ø> (ø)
internal/handlers/common/errorcode_string.go	80.00% <ø> (ø)
internal/handlers/common/msg_listcommands.go	85.71% <ø> (ø)
internal/handlers/dummy/msg_setfreemonitoring.go	0.00% <0.00%> (ø)
internal/handlers/tigris/msg_setfreemonitoring.go	0.00% <0.00%> (ø)
internal/handlers/common/msg_setfreemonitoring.go	76.00% <76.00%> (ø)
internal/handlers/pg/msg_setfreemonitoring.go	100.00% <100.00%> (ø)
internal/handlers/pg/msg_dbstats.go	72.09% <0.00%> (+6.97%)	⬆️

Flag	Coverage Δ
FerretDB	62.67% <77.77%> (+0.09%)	⬆️
MongoDB	7.22% <0.00%> (-0.03%)	⬇️
integration	62.77% <77.77%> (+0.08%)	⬆️
unit	25.51% <0.00%> (-0.10%)	⬇️

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

[AlekSi]

@rumyantseva @seeforschauer PTAL