default CompletableFuture<byte[]> collectBytes() {
        return collectBytes(defaultSubscriberExecutor());
    }
Collects the bytes published by this {@link ByteStreamMessage}.
The returned {@link CompletableFuture} will be notified when the elements are fully consumed.

<pre>{@code
StreamMessage<HttpData> source = StreamMessage.of(HttpData.wrap(new byte[] { 1, 2, 3 }),
                                                  HttpData.wrap(new byte[] { 4, 5, 6 }));

byte[] collected = ByteStreamMessage.of(source).collectBytes().join();
assert Arrays.equals(collected, new byte[] { 1, 2, 3, 4, 5, 6 });
}</pre>
core/src/main/java/com/linecorp/armeria/common/stream/ByteStreamMessage.java