refactor: refine foyer-experimental-bench structure

[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.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have passed make check and make test or make all in my local envirorment.

Related issues or PRs (optional)

[codecov]

Codecov Report

Attention: 140 lines in your changes are missing coverage. Please review.

Comparison is base (09b8b10) 75.38% compared to head (fcfded6) 74.03%.

Files	Patch %	Lines
foyer-experimental/src/metrics.rs	0.00%	114 Missing ⚠️
foyer-experimental/src/wal.rs	0.00%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #259      +/-   ##
==========================================
- Coverage   75.38%   74.03%   -1.36%     
==========================================
  Files          50       51       +1     
  Lines        6030     6140     +110     
==========================================
  Hits         4546     4546              
- Misses       1484     1594     +110     

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