filebeat+kafka,drop_fields。自定义传入kafka里的字段

filebeat将日志信息传入kafka，消费kafka的时候可以看到这样的结果
 {
     "@timestamp"  :  "2017-11-27T07:48:10.967Z",
     "@metadata"  :  {"beat":"filebeat","type":"doc","version":"6.0.0","topic":"beats"},
     "source"  :  "/var/log/secure","offset":13588,
     "message"  : "Nov 27 15:48:04 host-127-0-0-1 sshd[11654]: Connection closed by 0.0.0.0",
     "prospector" :  {"type":"log"},
     "beat"  :  {"name":"host-127-0-0-1","hostname":"host-127-0-0-1","version":"6.0.0"}
  }
我只想让日志信息"Nov 27 15:48:04 host-127-0-0-1 sshd[11654]: Connection closed by 0.0.0.0"直接传入kafka，为什么附加了那么多别的值？？？
我用drop_fields删字段，别的都可以删，@timestamp，@metadata这两个字段始终删除不了。。。求解？