perf(logger): use io.WriteString for log output

[huynhanx03]

Overview

Optimize logger write path in LoggerWithConfig by using io.WriteString for already-formatted log strings.

Change

• Replace fmt.Fprint(out, formatter(param)) with io.WriteString(out, formatter(param)).
• Keep defaultLogFormatter output unchanged.

Benchmark

goos: darwin
goarch: arm64
cpu: Apple M1

BenchmarkLoggerMiddleware
metric        old      new      delta
ns/op         1,688    761.9    -54.9%
B/op          252      236      -6.3%
allocs/op     9        8        -11.1%

Validation

• go test . -run 'TestDefaultLogFormatter|TestLoggerWithConfigFormatting'
• go test . -run=^$ -bench='BenchmarkLoggerMiddleware' -benchmem -benchtime=500ms

Checklist

• Open your pull request against the master branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.
• If the pull request introduces a new feature, the feature is documented in the docs/doc.md.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.37%. Comparing base (3dc1cd6) to head (eb2cf6f).
⚠️ Report is 276 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4652      +/-   ##
==========================================
- Coverage   99.21%   98.37%   -0.84%     
==========================================
  Files          42       48       +6     
  Lines        3182     3138      -44     
==========================================
- Hits         3157     3087      -70     
- Misses         17       42      +25     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.36% <100.00%> (?)
-tags go_json	98.29% <100.00%> (?)
-tags nomsgpack	98.35% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.37% <100.00%> (?)
go-1.26	98.37% <100.00%> (?)
macos-latest	98.37% <100.00%> (-0.84%)	⬇️
ubuntu-latest	98.37% <100.00%> (-0.84%)	⬇️

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[huynhanx03]

Hi @appleboy , could you review?