Add: gin.StyleLogger, and the method functions and unit test codes of it.

[kerbalwzy]

Add a new struct type use for output the custom log information in gin.HanlderFunc with gin log style.

Unit testing result:

Preview of Actual Use Effect:

Usage demo code:

package main

import (
	"github.com/gin-gonic/gin"
	"log"
)

var GlobalGinStyleLogger *gin.StyleLogger

func init() {
	GlobalGinStyleLogger = gin.NewGinStyleLogger(nil, nil)
}

func ginStylLoggerDemo(c *gin.Context) {
	GlobalGinStyleLogger.Fprintln(c, 500, "<custom log information here>")
	c.Status(200)
}

func main() {
	router := gin.Default()
	router.GET("/example", ginStylLoggerDemo)
	err := router.Run(":8080")
	if nil != err {
		log.Fatal(err)
	}
}

[codecov]

Codecov Report

Merging #2096 into master will decrease coverage by 0.41%.
The diff coverage is 76.74%.

@@            Coverage Diff            @@
##           master   #2096      +/-   ##
=========================================
- Coverage   98.92%   98.5%   -0.42%     
=========================================
  Files          40      40              
  Lines        2229    2272      +43     
=========================================
+ Hits         2205    2238      +33     
- Misses         12      18       +6     
- Partials       12      16       +4

Impacted Files	Coverage Δ
logger.go	93.54% <76.74%> (-6.46%)	⬇️

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[thinkerou]

@kerbalwzy thanks a lot! please see #1677