public LlapThreadLocalStatistics subtract(LlapThreadLocalStatistics other) {
    for (Map.Entry<String, LlapFileSystemStatisticsData> otherStats :
        other.schemeToThreadLocalStats.entrySet()){
      schemeToThreadLocalStats.computeIfPresent(otherStats.getKey(),
          (thisScheme, stats) -> stats.subtract(otherStats.getValue()));
    }

    cpuTime -= other.cpuTime;
    userTime -= other.userTime;

    return this;
  }
Merges the list to a map.
Input list:
1. FileSystem.Statistics (scheme: file)
2. FileSystem.Statistics (scheme: hdfs)
3. FileSystem.Statistics (scheme: hdfs)
Output map:
file : LlapThreadLocalStatistics.StatisticsData (1)
hdfs : LlapThreadLocalStatistics.StatisticsData (2 + 3)
llap-common/src/java/org/apache/hadoop/hive/llap/LlapThreadLocalStatistics.java