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