@Override
        public void onComplete() {
            final long produced = this.produced;
            if (produced != 0L) {
                this.produced = 0L;
                produced(produced);
            }
            nextSource();
        }
Relay items in order from subsequent {@link StreamMessage}s as a single {@link StreamMessage} source.
core/src/main/java/com/linecorp/armeria/common/stream/ConcatArrayStreamMessage.java