default CompletableFuture<Void> subscribe() {
        return subscribe(defaultSubscriberExecutor());
    }
Drains and discards all objects in this {@link StreamMessage}.

<p>For example:<pre>{@code
StreamMessage<Integer> source = StreamMessage.of(1, 2, 3);
List<Integer> collected = new ArrayList<>();
CompletableFuture<Void> future = source.peek(collected::add).subscribe();
future.join();
assert collected.equals(List.of(1, 2, 3));
assert future.isDone();
}</pre>
core/src/main/java/com/linecorp/armeria/common/stream/StreamMessage.java