Uggh: not beneficial, IIRC Graveler RocksDB tables do not
have protobuf values. Instead they use some varint encoding, and hold a buffer of bytes in the value. That
ends up as a protobuf, but I don't think it is exposed as an actual Spark column before we finish decoding.
But a great opportunity to see whether our shading rules are good enough to separate "their" protobuf from "our" protobuf. We shall see once we start supporting 3.4.