@Override
        public String toString() {
            return MoreObjects.toStringHelper(Subscription.class)
                              .add("publisher", publisher)
                              .add("demand", publisher.demand())
                              .add("executor", executor)
                              .add("options", options).toString();
        }
Replaces the subscriber with a placeholder so that it can be garbage-collected and
we conform to the Reactive Streams specification rule 3.13.
core/src/main/java/com/linecorp/armeria/common/stream/CancellableStreamMessage.java