docs(fields): add custom fields example

#18
#22
#24

README.md

@@ -118,3 +118,50 @@ This will add the `traceId` field to log:
   "traceID": "285f31ec1dba4b79034c4415ad18e4ed"
 }
 ```
+
+## Custom Zap fields
+
+example for custom log request body and response request ID
+
+```go
+func main() {
+  r := gin.New()
+
+  logger, _ := zap.NewProduction()
+
+  r.Use(ginzap.GinzapWithConfig(logger, &ginzap.Config{
+    UTC:        true,
+    TimeFormat: time.RFC3339,
+    Context: ginzap.Fn(func(c *gin.Context) (fields []zapcore.Field) {
+      // log response ID
+      if requestID := c.Writer.Header().Get("X-Request-Id"); requestID != "" {
+        fields = append(fields, zap.String("request-id", requestID))
+      }
+
+      // log request body
+      var body []byte
+      var buf bytes.Buffer
+      tee := io.TeeReader(c.Request.Body, &buf)
+      body, _ = io.ReadAll(tee)
+      c.Request.Body = io.NopCloser(&buf)
+      fields = append(fields, zap.String("body", string(body)))
+
+      return
+    }),
+  }))
+
+  // Example ping request.
+  r.GET("/ping", func(c *gin.Context) {
+    c.Writer.Header().Add("X-Request-Id", "1234-5678-9012")
+    c.String(200, "pong "+fmt.Sprint(time.Now().Unix()))
+  })
+
+  r.POST("/ping", func(c *gin.Context) {
+    c.Writer.Header().Add("X-Request-Id", "9012-5678-1234")
+    c.String(200, "pong "+fmt.Sprint(time.Now().Unix()))
+  })
+
+  // Listen and Server in 0.0.0.0:8080
+  r.Run(":8080")
+}
+```

_example/example02/main.go

@@ -0,0 +1,54 @@
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"time"
+
+	ginzap "github.com/gin-contrib/zap"
+	"github.com/gin-gonic/gin"
+	"go.uber.org/zap"
+	"go.uber.org/zap/zapcore"
+)
+
+func main() {
+	r := gin.New()
+
+	logger, _ := zap.NewProduction()
+
+	r.Use(ginzap.GinzapWithConfig(logger, &ginzap.Config{
+		UTC:        true,
+		TimeFormat: time.RFC3339,
+		Context: ginzap.Fn(func(c *gin.Context) (fields []zapcore.Field) {
+			// log response ID
+			if requestID := c.Writer.Header().Get("X-Request-Id"); requestID != "" {
+				fields = append(fields, zap.String("request-id", requestID))
+			}
+
+			// log request body
+			var body []byte
+			var buf bytes.Buffer
+			tee := io.TeeReader(c.Request.Body, &buf)
+			body, _ = io.ReadAll(tee)
+			c.Request.Body = io.NopCloser(&buf)
+			fields = append(fields, zap.String("body", string(body)))
+
+			return
+		}),
+	}))
+
+	// Example ping request.
+	r.GET("/ping", func(c *gin.Context) {
+		c.Writer.Header().Add("X-Request-Id", "1234-5678-9012")
+		c.String(200, "pong "+fmt.Sprint(time.Now().Unix()))
+	})
+
+	r.POST("/ping", func(c *gin.Context) {
+		c.Writer.Header().Add("X-Request-Id", "9012-5678-1234")
+		c.String(200, "pong "+fmt.Sprint(time.Now().Unix()))
+	})
+
+	// Listen and Server in 0.0.0.0:8080
+	r.Run(":8080")
+}