@Override
    public List<Pair<Cell, Cell>> postAppendBeforeWAL(
      ObserverContext<? extends RegionCoprocessorEnvironment> ctx, Mutation mutation,
      List<Pair<Cell, Cell>> cellPairs) throws IOException {
      return cellPairs.stream()
        .map(pair -> new Pair<>(pair.getFirst(), newCellWithNotExistColumnFamily(pair.getSecond())))
        .collect(Collectors.toList());
    }
Test coprocessor methods
{@link RegionObserver#postIncrementBeforeWAL(ObserverContext, Mutation, List)} and
{@link RegionObserver#postAppendBeforeWAL(ObserverContext, Mutation, List)}. These methods may
change the cells which will be applied to memstore and WAL. So add unit test for the case which
change the cell's column family and tags.
hbase-server/src/test/java/org/apache/hadoop/hbase/coprocessor/TestPostIncrementAndAppendBeforeWAL.java