@Override
  public void updateUnblockedFlushCount(long unblockedFlushCount) {
    if (unblockedFlushCount > 0) {
      unblockedFlushHistogram.add(unblockedFlushCount);
      unblockedFlushGauge.set(unblockedFlushCount);
    }
  }
Hadoop2 implementation of MetricsHeapMemoryManagerSource. Implements BaseSource through
BaseSourceImpl, following the pattern
hbase-hadoop-compat/src/main/java/org/apache/hadoop/hbase/regionserver/MetricsHeapMemoryManagerSourceImpl.java