public StreamObserver<BeamFnApi.Elements> getOutboundObserver() {
    return outboundObserver;
  }
@Test
  public void testOutboundObserver() {
    Collection<BeamFnApi.Elements> values = new ArrayList<>();
    BeamFnDataGrpcMultiplexer multiplexer =
        new BeamFnDataGrpcMultiplexer(
            DESCRIPTOR,
            OutboundObserverFactory.clientDirect(),
            inboundObserver -> TestStreams.withOnNext(values::add).build());
    multiplexer.getOutboundObserver().onNext(ELEMENTS);
    MatcherAssert.assertThat(values, Matchers.contains(ELEMENTS));
  }