refactor: use us instead of ns for tracing

[MrCroxx]

What's changed and what's your intention?

Please explain IN DETAIL what the changes are in this PR and why they are needed. :D

As title. Why on earth I chose ns as default. = =

Checklist

• I have written the necessary rustdoc comments
• I have added the necessary unit tests and integration tests
• I have passed make all (or make fast instead if the old tests are not modified) in my local environment.

Related issues or PRs (optional)

#577

[codecov]

Codecov Report

Attention: Patch coverage is 74.19355% with 8 lines in your changes missing coverage. Please review.

Project coverage is 86.20%. Comparing base (fe904bc) to head (aef1bf4).
Report is 1 commits behind head on main.

Files	Patch %	Lines
foyer-bench/src/main.rs	66.66%	5 Missing ⚠️
foyer/src/hybrid/cache.rs	72.72%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #582   +/-   ##
=======================================
  Coverage   86.20%   86.20%           
=======================================
  Files          59       59           
  Lines        7597     7597           
=======================================
  Hits         6549     6549           
  Misses       1048     1048           

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