glg.go
func WarnFunc(f func() string) error {
	if isModeEnable(WARN) {
		return glg.out(WARN, "%s", f())
	}
	return nil
}
func WarnFunc(f func() string) error {
	if isModeEnable(WARN) {
		return glg.out(WARN, "%s", f())
	}
	return nil
}
// WarnFunc outputs Warn level log returned from the function