Why the meta in registry file is always null?

I'm using a log input for my filebeat6.7, i get the registry file like this:

  [
    {
        "source": "/opt/log/test.log",
        "offset": 916,
        "timestamp": "2020-05-22T08:58:13.876693008Z",
        "ttl": -1,
        "type": "log",
        "meta": null,
        "FileStateOS": {
            "inode": 1111111,
            "device": 11115
        }
    }
]

why the "meta" is null ? can i configure it ?