feature/faces: use pooler face instead pool struct; up coverage and init tests for BucketStat; add Stringer methods for rs info

[KaymeKaydex]

• use face instead pool struct
• add mocks and first test with mock for replicaset bucket stat
• make bucket stat public method(there is no problem if user wanna use it)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 16.20%. Comparing base (9e72b8c) to head (b2cf45c).

Files	Patch %	Lines
discovery.go	0.00%	1 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   13.82%   16.20%   +2.38%     
==========================================
  Files           9        9              
  Lines         615      617       +2     
==========================================
+ Hits           85      100      +15     
+ Misses        525      511      -14     
- Partials        5        6       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.