log: avoid setting default slog logger in init

[ucwong]

slog.SetDefault has undesirable side effects. It also sets the default logger destination, for example. So we should not call it by default in init.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (635bb4d) 52.80% compared to head (f86cc65) 52.78%.
Report is 444 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1877      +/-   ##
==========================================
- Coverage   52.80%   52.78%   -0.02%     
==========================================
  Files         493      493              
  Lines       60529    60528       -1     
==========================================
- Hits        31960    31949      -11     
- Misses      26132    26137       +5     
- Partials     2437     2442       +5     

Files	Coverage Δ
log/root.go	8.69% <100.00%> (-20.48%)	⬇️

... and 14 files with indirect coverage changes