public MemStoreSize getMemStoreSize() {
    return new MemStoreSize(getDataSize(), getHeapSize(), getOffHeapSize(), getCellsCount());
  }
Accounting of current heap and data sizes. Thread-safe. Many threads can do updates against this
single instance.
@see NonThreadSafeMemStoreSizing
@see MemStoreSize
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/ThreadSafeMemStoreSizing.java