armeria - Accessing response headers using a decorator in Armeria

Question

I would like to add a decorator to my armeria client that checks each http response if a certain http header was returned:

    builder.decorator((delegate, ctx, req) -> {
      final HttpResponse response = delegate.execute(ctx, req);
      final AggregatedHttpResponse r = response.aggregate().join();
      for (Map.Entry<AsciiString, String> header : r.headers()) {
        if ("warning".equalsIgnoreCase(header.getKey().toString())) {
          throw new IllegalArgumentException("Detected usage of deprecated API for request "
            + req.toString() + ":\n" + header.getValue());
        }
      }
      return response;
    });

However, when starting my client it blocks on the join() call and waits forever. Is there a standard pattern for this in Armeria ? Presumably i cannot just block on the response in an interceptor, but i could not find a way to access the response headers otherwise. Using subscribe or toDuplicator did not work any better though.