code: public synchronized Headers trailers() throws IOException {
      if (errorCode != null) {
        throw errorException != null ? errorException : new StreamResetException(errorCode);
      }
      if (!source.finished || !source.receiveBuffer.exhausted() || !source.readBuffer.exhausted()) {
        throw new IllegalStateException("too early; can't read the trailers yet");
      }
      return source.trailers != null ? source.trailers : Util.EMPTY_HEADERS;
    }
docs: Returns the trailers. It is only safe to call this once the source stream has been completely
  exhausted.