private void updateMetricsStore(boolean memstoreRead) {
    if (store != null) {
      store.updateMetricsStore(memstoreRead);
    } else {
      // for testing.
      if (memstoreRead) {
        memstoreOnlyReads++;
      } else {
        mixedReads++;
      }
    }
  }
Increment the metric if all the cells are from memstore. If not we will account it
for mixed reads
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/StoreScanner.java