RFC 5424定义了八种日志级别：emergency ，alert ，critical, error ，warning ，notice ，info 和 debug 。
0 Emergency: system is unusable
1 Alert: action must be taken immediately
2 Critical: critical conditions
3 Error: error conditions
4 Warning: warning conditions
5 Notice: normal but significant condition
6 Informational: informational messages
7 Debug: debug-level messages
参考 RFC 5424