@Override
  public Optional<RegionObserver> getRegionObserver() {
    // Extremely important to be sure that the coprocessor is invoked as a RegionObserver
    return Optional.of(this);
  }
A RegionObserver which modifies incoming Scan requests to include additional columns than what
the user actually requested.
hbase-examples/src/main/java/org/apache/hadoop/hbase/coprocessor/example/ScanModifyingObserver.java