### 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
try (Directory directory = newDirectory()) {
            RandomIndexWriter indexWriter = new RandomIndexWriter(random(), directory);
            for (int i = 0; i < 10; i++) {
                indexWriter.addDocument(List.of(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);
                int segmentsWithLiveDocs = (int) searcher.getIndexReader()
                    .leaves()
                    .stream()
                    .map(LeafReaderContext::reader)
                    .filter(leafReader -> leafReader.getLiveDocs() != null)
                    .count();
                debugTestCase(
                    builder,
                    new MatchAllDocsQuery(),
                    searcher,