public long getCurrentObsWindowTs() {
      return Math.min(timestamp.get() - monitorStartTime, timestamp.get() - startObsWindowTs.get());
    }
Returns the length of current observation window, usually equal to OBSERVATION_WINDOW_MS. If
IoTDB is started after the start of the theoretical time window, then IoTDB startup time is
returned.

@return current observation window time, millisecond.
iotdb-core/node-commons/src/main/java/org/apache/iotdb/commons/service/metric/JvmGcMonitorMetrics.java