code: public Http2Stream pushStream(int associatedStreamId, List<Header> requestHeaders, boolean out)
        throws IOException {
      if (client) throw new IllegalStateException("Client cannot push requests.");
      return newStream(associatedStreamId, requestHeaders, out);
    }
docs: Returns a new server-initiated stream.
  
  @param associatedStreamId the stream that triggered the sender to create this stream.
  @param out true to create an output stream that we can use to send data to the remote peer.
  Corresponds to {@code FLAG_FIN}.