public ServerSideScanMetrics getMetrics() {
    assert isTrackingMetrics();
    return this.metrics;
  }
Get the metrics instance. Should only be called after a call to {@link #isTrackingMetrics()}
has been made to confirm that metrics are indeed being tracked.
@return {@link ServerSideScanMetrics} instance that is tracking metrics for this scan
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/ScannerContext.java