Oh that’s interesting - looks like you’re trying to sample the data using a javascript function? not seen that done before! 
Have you enabled javascript on your cluster?
Also, did you try explicitly naming dimensions in the dimensionsSpec?
OOI did you work this out?
I had in my mind that there was some issue with the dimensions in DruidInputSource ingestion … but that could have been brainmush 
(Also, say hi in Druid FOrum if you’re in there )