I poked around protobuf code. Do we want the wire size of Result-s, in particular?
Getting that out of the protobuf doesn't look possible via normal interfaces. We can call getSerializedSize on each Result but that may be expensive. We can also call getSerializedSize on them on server side, before adding them to response, and store that inside response too (one long per response); getSerializedSize is memoized on first call, it's called recursively on nested structures, and called during message serialization in RPC (and during write by protobuf), this call is made anyway and thus should not impact perf.