Unfortunately, we don't currently have any support for this kind of zero-copy read on the receiving side. In the long run, we have some ideas about how we'd like to support this kind of thing, but we haven't had time to turn our attention to this yet, so we have no concrete plan or ETA yet. But I would expect that we will eventually add some APIs to support this kind of thing.
Note that gRPC does not provide any guarantee that received messages will show up in any particular chunk size. You may happen to be seeing 8192 bytes right now, but that's highly dependent upon the server application, the network, proxies, encryption software, and a bunch of other factors, so what you're seeing right now could change at any time.