Github user sardell commented on the issue:
https://github.com/apache/metron/pull/1010
@justinleet I've changed the property to be `source.type.field` and added
the property to the documentation at
https://github.com/apache/metron/tree/master/metron-platform/metron-common#global-configuration.
I was slightly confused about the subset field, but I took an educated guess
and entered `UI` as the value. Let me know if this is not correct and I'll
update it.---
