pkg/otel/arrow_record/{ConsumerAPI,ProducerAPI} is not idiomatic naming

[jmacd]

When I introduced gomock support in pkg/otel/arrow_record, I created two new interfaces used only in testing. Since the names Consumer and Producer (both structs) were taken, I created *API types. I also considered Iface. I could move these into a test package, or we can make the real types unexported and only export the interface. Something should be done.