add example for ctx keys feature of package glog (#3273)

example/os/log/ctx_keys/config.yaml

@@ -0,0 +1,4 @@
+logger:
+  Level:   "all"
+  Stdout:  true
+  CtxKeys: ["RequestId", "UserId"]

example/os/log/ctx_keys/main.go

@@ -0,0 +1,20 @@
+// Copyright GoFrame Author(https://goframe.org). All Rights Reserved.
+//
+// This Source Code Form is subject to the terms of the MIT License.
+// If a copy of the MIT was not distributed with this file,
+// You can obtain one at https://github.com/gogf/gf.
+
+package main
+
+import (
+	"context"
+
+	"github.com/gogf/gf/v2/frame/g"
+)
+
+func main() {
+	var ctx = context.Background()
+	ctx = context.WithValue(ctx, "RequestId", "123456789")
+	ctx = context.WithValue(ctx, "UserId", "10000")
+	g.Log().Error(ctx, "runtime error")
+}

util/gvalid/gvalid_validator_check_value.go

@@ -193,6 +193,7 @@ func (v *Validator) doCheckValue(ctx context.Context, in doCheckValueInput) Erro
 				}
 				// The error should have error code that is `gcode.CodeValidationFailed`.
 				if gerror.Code(err) == gcode.CodeNil {
+					// TODO it's better using interface?
 					if e, ok := err.(*gerror.Error); ok {
 						e.SetCode(gcode.CodeValidationFailed)
 					}