Currently if you provide an ANN search using the `knn` search section, the search doesn't respect cancellation. This is because ANN searches use their own Lucene data structures that aren't covered by `ExitableDirectoryReader`.

This class is forked from Lucene, and in Lucene's version we recently added fine-grained cancellation for ANN search (https://github.com/apache/lucene/pull/833). So I think we just need to fold in the changes to the Elasticsearch version.
server/src/main/java/org/elasticsearch/search/internal/ExitableDirectoryReader.java
queryCancellation.checkCancelled();
            return in.getMaxPackedValue();
        }
        @Override
        public int getNumDimensions() throws IOException {
            queryCancellation.checkCancelled();
            return in.getNumDimensions();
        }
        @Override
        public int getNumIndexDimensions() throws IOException {
            queryCancellation.checkCancelled();
            return in.getNumIndexDimensions();
        }
        @Override
        public int getBytesPerDimension() throws IOException {
            queryCancellation.checkCancelled();
            return in.getBytesPerDimension();
        }
        @Override
        public long size() {
            queryCancellation.checkCancelled();
            return in.size();
        }
        @Override
        public int getDocCount() {
            queryCancellation.checkCancelled();
            return in.getDocCount();
        }
    }
    private static class ExitablePointTree implements PointValues.PointTree {