### 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
* Segments with a filter that doesn't rewrite to {@code match_all} can't
     * take the fast path. But segments who's filter rewrites to {@code match_all}
     * can use the fast path - thus the assertion at the bottom of this:
     * {@code "segments_counted_in_constant_time", lessThan(searcher.getLeafContexts().size())}.
     */
    public void testTermOnFilteredIndex() throws IOException {
        KeywordFieldType ft = new KeywordFieldType("foo");
        AggregationBuilder builder = new FiltersAggregationBuilder("test", new KeyedFilter("q1", new TermQueryBuilder("foo", "bar")));
        try (Directory directory = newDirectory()) {
            RandomIndexWriter indexWriter = new RandomIndexWriter(random(), directory);
            for (int i = 0; i < 10; i++) {
                indexWriter.addDocument(
                    List.of(new Field("foo", new BytesRef("bar"), KeywordFieldMapper.Defaults.FIELD_TYPE), new LongPoint("t", i))
                );
            }
            indexWriter.close();
            try (DirectoryReader directoryReader = DirectoryReader.open(directory)) {
                final IndexSettings indexSettings = createIndexSettings();
                BitsetFilterCache bitsetFilterCache = new BitsetFilterCache(indexSettings, new BitsetFilterCache.Listener() {
                    @Override
                    public void onRemoval(ShardId shardId, Accountable accountable) {}
                    @Override
                    public void onCache(ShardId shardId, Accountable accountable) {}
                });
                DirectoryReader limitedReader = new DocumentSubsetDirectoryReader(
                    ElasticsearchDirectoryReader.wrap(directoryReader, new ShardId(indexSettings.getIndex(), 0)),
                    bitsetFilterCache,
                    LongPoint.newRangeQuery("t", 5, Long.MAX_VALUE)
                );
                IndexSearcher searcher = newIndexSearcher(limitedReader);