Cloud Bigtable's client API differs from it's protocol. So it requires a manual wrapper and uses GAPIC as an internal implementation detail. Unfortunately this means that the underlying gapic api is exposed alongside the intended api. This is not only confusing to customers, but also forces the client developers to keep the gapic surface stable. To avoid this issue I'm currently using synth.py script to mutate the autogenerated to hide it:
googleapis/google-cloud-java#5035

It would be nice if there was a way to do this without this hack