void nextSource() {
            if (!cancelled()) {
                final int index = this.index;
                if (index == sources.size()) {
                    downstream.onComplete();
                } else {
                    this.index++;
                    sources.get(index).subscribe(this, executor, options);
                }
            }
        }
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