I see that the Beam codebase includes a PubsubGrpcClient, but there doesn't appear to be any way to configure PubsubIO to use that client over the PubsubJsonClient.
There's even a PubsubIO.Read#withClientFactory, but it's marked as for testing only. Is gRPC support something that's still in development? Or am I missing something about how to configure this? I'm particularly interested in using gRPC due to the message size inflation of base64 encoding required for JSON transport. My payloads are all below the 10 MB Pubsub limit, but I need to support some near the top end of that range that are currently causing errors due to base64 inflation.