log/logger.go
func (l *Logger) Warnf(format string, args ...interface{}) {
	l.Output(2, LevelWarn, fmt.Sprintf(format, args...))
}
func (l *Logger) Warnf(format string, args ...interface{}) {
	l.Output(2, LevelWarn, fmt.Sprintf(format, args...))
}
// Warnf records the log with warn level