@Override
  public ScanMetrics getScanMetrics() {
    return scanMetrics;
  }
Used internally accumulating metrics on scan. To enable collection of metrics on a Scanner,
call {@link Scan#setScanMetricsEnabled(boolean)}.
@return Returns the running {@link ScanMetrics} instance or null if scan metrics not enabled.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/AbstractClientScanner.java