chore: share UnitToDuration logic

[zirain]

xref: #4495

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.

Project coverage is 65.60%. Comparing base (9353be2) to head (df7b07b).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/utils/ratelimit/unit.go	88.88%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4510   +/-   ##
=======================================
  Coverage   65.60%   65.60%           
=======================================
  Files         210      211    +1     
  Lines       31612    31602   -10     
=======================================
- Hits        20739    20733    -6     
+ Misses       9670     9666    -4     
  Partials     1203     1203           

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