For the APIs which support both REST and gRPC, how should we expose the choice? One option would be to have a different Connection class for each endpoint: if it works, then after instantiating the desired client, calling code would in theory be happy without caring again which one was chosen. Thoughts?

Because the datastore API is protobufs-over-REST, its Connection class already has a bunch of domain-specific API calls already, which would make this plan simpler. the Connection classes for pubsub and logging expose only make_api_request, which won't be much help.