private void init() {
            values = new LinkedList<>();
            for (int i = 0; i < size; i++) {
                values.add(null);
            }
        }
A metrics window, based on AlarmRule#period. This window slides with time, just keeps the recent N(period)
buckets.
oap-server/server-alarm-plugin/src/main/java/org/apache/skywalking/oap/server/core/alarm/provider/RunningRule.java