zapslog: Support for a custom mapping of slog.Level to zap.Level

[arukiidou]

• Refs zapslog stabilization tracking issue #1333
• Resolves zapslog: Support for a custom mapping of slog.Level to zap.Level #1412

summary

• enables custom mapping of slog.Level to zap.Level .
• If zapslog.WithConvertLeveler not given, the behavior is not changed.

tests

• passed test Feature/custom mapping level arukiidou/zap#4

[arukiidou]

@sywhang please take a look?

[zekth]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.63%. Comparing base (8f5ee80) to head (5a63490).
Report is 32 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1413   +/-   ##
=======================================
  Coverage   98.63%   98.63%           
=======================================
  Files          53       54    +1     
  Lines        2997     3002    +5     
=======================================
+ Hits         2956     2961    +5     
  Misses         32       32           
  Partials        9        9           

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[arukiidou]

The reason for drafting this
Because I think it may be better to replace it with an implementation of slog.HandlerOptions#ReplaceAttr.

What do you think?

https://pkg.go.dev/log/slog#example-HandlerOptions-CustomLevels