Optimize the logger middleware

[mstmdev]

Use the out.Write to write the log content immediately, because the LogFormatter returns a string, the fmt.Fprint is unnecessary here.

[mstmdev]

The benchmark test result:

Command:

go test -test.v -test.bench ^BenchmarkLoggerMiddleware$

Environment:

version: go1.18.3
goos: windows
goarch: amd64
pkg: github.com/gin-gonic/gin
cpu: Intel(R) Core(TM) i7-10700 CPU @ 2.90GHz

Result:

BenchmarkLoggerMiddleware(New Pull Request)
BenchmarkLoggerMiddleware-16             1244007               951.5 ns/op           270 B/op         14 allocs/op

BenchmarkLoggerMiddleware(Old Master)
BenchmarkLoggerMiddleware-16             1000000              1036 ns/op             288 B/op         16 allocs/op

[codecov[bot]]

Codecov Report

Merging #3016 (8dbee16) into master (92dd245) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3016   +/-   ##
=======================================
  Coverage   98.06%   98.06%           
=======================================
  Files          43       43           
  Lines        3148     3150    +2     
=======================================
+ Hits         3087     3089    +2     
  Misses         48       48           
  Partials       13       13           

Flag	Coverage Δ
go-1.15	98.06% <100.00%> (+<0.01%)	:arrow_up:
go-1.16	98.03% <100.00%> (+<0.01%)	:arrow_up:
go-1.17	97.96% <100.00%> (+<0.01%)	:arrow_up:
go-1.18	97.96% <100.00%> (+<0.01%)	:arrow_up:
macos-latest	98.06% <100.00%> (+<0.01%)	:arrow_up:
nomsgpack	98.03% <100.00%> (+<0.01%)	:arrow_up:
ubuntu-latest	98.06% <100.00%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
logger.go	100.00% <100.00%> (ø)

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