@Override
  public Snapshot getSnapshot() {
    removeExpireValues();
    return new Snapshot(storage.values());
  }
Return the new updating time. If the new value's system time equals to last
value's, use the last updating time + 1 as the new updating time. This
operation guarantees all the updating times in the <code>storage</code>
strictly increment. No override happens before reaching the
<code>collisionBuffer</code>.

@return the updating time
samza-api/src/main/java/org/apache/samza/metrics/SlidingTimeWindowReservoir.java