Feaure/add status override from context

[zoftdev]

This feature is about to override code label base on context value.

use StatusOverrideFromContext

p := ginprometheus.NewPrometheus("gin")
p.StatusOverrideFromContext = append(p.StatusOverrideFromContext, "code")
p.StatusOverrideFromContext = append(p.StatusOverrideFromContext, "mycode")

and set gin's context

        r.GET("/", func(c *gin.Context) {
		c.Set("code", "888")
		c.JSON(200, "Hello world!")
	})

See example at here