As I said, I would ask a new question about Java::OrgLogstash::MissingConverterException. You need to get the input working before you worry about the filter.
A PGobject is used to represent an unknown type, so the input does not know what to do with it. It's probably related to the fact that the column contains JSON. Perhaps you need to CAST the column (because you want a string to input to the json filter). I don't know. I don't use the jdbc input.