private static Duration addDurations(Duration a, Duration b) {
      return new Duration(a.getValue(NANOSECONDS) + b.getValue(NANOSECONDS), NANOSECONDS);
    }
this class is copied from
https://github.com/airlift/airlift/blob/214/stats/src/main/java/io/airlift/stats/CpuTimer.java as
it doesn't support Java8.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/utils/stats/CpuTimer.java