input without 'message' requires 'source' to be set

[tomryanx]

When using the json codec, or otherwise not having a message field, it appears to me that the fingerprint filter will quietly fail to produce anything unless source is set.

Perhaps the documentation could indicate that whenever a message field is not present, for example when using a json codec (if that is indeed my issue), that source must be explicitly configured. I understand that it's not a required field, but a hint would be great.

This occurs for me shipping pre-structured json with filebeat to a beats input with json codec.