docs: improve API performance and testing robustness

- Add a comment describing the `Fn` function which retrieves zap fields from `gin.Context`

Signed-off-by: appleboy <[email protected]>

zap.go

@@ -17,6 +17,7 @@ import (
 	"go.uber.org/zap/zapcore"
 )
 
+// Fn is a function to get zap fields from gin.Context
 type Fn func(c *gin.Context) []zapcore.Field
 
 // Skipper is a function to skip logs based on provided Context