### Description

It'd be lovely is a `filters` or `filter` agg that touches a `constant_keyword` would properly rewrite itself to `match_all` and `match_none` and "go fast", at least when it's at the root of the aggs tree. Theoretically it should "just work" but it doesn't seem to. Also! The `terms` agg should be able to act on there being only a single possible value. Maybe it can even act on the `match_all`/`match_none` nature of the `constant_keyword`. I'm not sure how, but it does feel like there are optimizations in there. Particularly when `terms` is the top level agg.
server/src/test/java/org/elasticsearch/search/aggregations/bucket/filter/FiltersAggregatorTests.java
}
    public void testKeyedFilter() throws Exception {
        Directory directory = newDirectory();
        RandomIndexWriter indexWriter = new RandomIndexWriter(random(), directory);
        Document document = new Document();
        document.add(new Field("field", new BytesRef("foo"), KeywordFieldMapper.Defaults.FIELD_TYPE));
        indexWriter.addDocument(document);
        document.clear();
        document.add(new Field("field", new BytesRef("else"), KeywordFieldMapper.Defaults.FIELD_TYPE));
        indexWriter.addDocument(document);
        
        indexWriter.commit();
        document.add(new Field("field", new BytesRef("foo"), KeywordFieldMapper.Defaults.FIELD_TYPE));
        indexWriter.addDocument(document);
        document.clear();
        document.add(new Field("field", new BytesRef("bar"), KeywordFieldMapper.Defaults.FIELD_TYPE));
        indexWriter.addDocument(document);
        document.clear();
        document.add(new Field("field", new BytesRef("foobar"), KeywordFieldMapper.Defaults.FIELD_TYPE));
        indexWriter.addDocument(document);
        indexWriter.commit();
        document.clear();
        document.add(new Field("field", new BytesRef("something"), KeywordFieldMapper.Defaults.FIELD_TYPE));
        indexWriter.addDocument(document);
        indexWriter.commit();
        document.clear();
        document.add(new Field("field", new BytesRef("foobar"), KeywordFieldMapper.Defaults.FIELD_TYPE));
        indexWriter.addDocument(document);
        indexWriter.close();
        DirectoryReader indexReader = DirectoryReader.open(directory);