public synchronized MetricCollector init(HiveConf conf) {
    //Can initialize the cache only once with a value.
    if (!isInited) {
      maxSize = getMaxSize(conf);
      isInited = true;
    }
    return instance;
  }
MetricCollector.
In memory collection of metrics
ql/src/java/org/apache/hadoop/hive/ql/parse/repl/metric/MetricCollector.java