@Override
        public String toString() {
            final ToStringHelper toStringHelper = toStringHelper(this);
            final Object[] elements = this.elements;
            if (elements != null) {
                toStringHelper.add("elements.length", elements.length);
            }

            return toStringHelper.add("head", head)
                                 .add("tail", tail)
                                 .add("size", size)
                                 .add("headOffset", headOffset)
                                 .add("lastRemovalRequestedOffset", lastRemovalRequestedOffset)
                                 .toString();
        }
Invoked by the executor in {@link DownstreamSubscription}.
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessageDuplicator.java