* designed for this nonstandard interaction will use it. As of 2019-01, the only widely-used
   * implementation of this pattern is [gRPC][grpc].
   *
   * Because the encoding of interleaved data is not well-defined for HTTP/1, duplex request
   * bodies may only be used with HTTP/2. Calls to HTTP/1 servers will fail before the HTTP request
   * is transmitted. If you cannot ensure that your client and server both support HTTP/2, do not