refactor: prevent unintentional breaking change after TracingChannel support

[wellwelwel]

Resolve the minor point in #4178 (review) related to a potential breaking change.

Note

The uncovered lines were not covered before implementation.

[codecov]

Codecov Report

❌ Patch coverage is 76.08696% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.58%. Comparing base (5b61d86) to head (a52b6f1).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/base/pool.js	76.08%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4182      +/-   ##
==========================================
- Coverage   90.60%   90.58%   -0.02%     
==========================================
  Files          87       87              
  Lines       14280    14281       +1     
  Branches     1798     1798              
==========================================
- Hits        12938    12937       -1     
- Misses       1342     1344       +2     

Flag	Coverage Δ
compression-0	89.84% <76.08%> (+0.16%)	⬆️
compression-1	90.56% <76.08%> (+2.41%)	⬆️
static-parser-0	88.27% <76.08%> (+0.90%)	⬆️
static-parser-1	89.00% <76.08%> (+0.20%)	⬆️
tls-0	90.02% <76.08%> (+3.21%)	⬆️
tls-1	90.37% <76.08%> (-0.02%)	⬇️

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