Consumer client ids defaulting to consumer-null-NNNN

[IanVaughan]

I'm trying to trace down a rouge consumer that connects with an odd "default" client id, eg consumer-null-2106
The reason is that we are losing messages, and its my guess they are going to these rouge consumers.

All our clients set a client_id, so I don't understand how this occurs:

They also have no consumer group id, which we set on all our connections:

I understand this is most likely not in racecar, and could be in rdkafka-ruby or lower down, but as this is the first entry point in the code I thought I would ask here first.