@Override
    public String toStorageData() {
        StringBuilder builder = new StringBuilder();

        this.data.forEach(element -> {
            if (builder.length() != 0) {
                // For the first element.
                builder.append(Const.ARRAY_SPLIT);
            }
            builder.append(element);
        });
        return builder.toString();
    }
IntList is a serializable array list carrying int values.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/analysis/metrics/IntList.java