public Tuple2<Long, Double> pop(String name, ImmutableMap<String, String> labels, Double value, long now) {
        ID id = new ID(name, labels);

        Tuple2<Long, Double> element = Tuple.of(now, value);
        Tuple2<Long, Double> result = lastElementMap.put(id, element);
        if (result == null) {
            return element;
        }
        return result;
    }
CounterWindow stores a series of counter samples in order to calculate the increase
or instant rate of increase.
oap-server/analyzer/meter-analyzer/src/main/java/org/apache/skywalking/oap/meter/analyzer/dsl/counter/CounterWindow.java