### 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
};
        
        FieldNamesFieldMapper.FieldNamesFieldType fnft = FieldNamesFieldMapper.FieldNamesFieldType.get(true);
        withAggregator(builder, new MatchAllDocsQuery(), buildIndex, (searcher, aggregator) -> {
            assertThat(aggregator, instanceOf(FilterByFilterAggregator.class));
            Map<String, Object> debug = collectAndGetFilterDebugInfo(searcher, aggregator);
            assertMap(debug, matchesMap().extraOk().entry("segments_counted_in_constant_time", greaterThan(0)));
        }, fieldType, fnft);
        testCase(buildIndex, (InternalFilters result) -> {
            assertThat(result.getBuckets(), hasSize(1));
            assertThat(result.getBucketByKey("q1").getDocCount(), equalTo(0L));
        }, new AggTestConfig(builder, fieldType, fnft));
    }
    @Override
    protected List<ObjectMapper> objectMappers() {
        return MOCK_OBJECT_MAPPERS;
    }
    private Map<String, Object> collectAndGetFilterDebugInfo(IndexSearcher searcher, Aggregator aggregator) throws IOException {
        aggregator.preCollection();
        for (LeafReaderContext ctx : searcher.getIndexReader().leaves()) {
            LeafBucketCollector leafCollector = aggregator.getLeafCollector(new AggregationExecutionContext(ctx, null, null, null));
            assertTrue(leafCollector.isNoop());
        }
        Map<String, Object> debug = new HashMap<>();
        ((FilterByFilterAggregator) aggregator).filters().get(0).collectDebugInfo(debug::put);
        return debug;
    }
    static final List<ObjectMapper> MOCK_OBJECT_MAPPERS = List.of(NestedAggregatorTests.nestedObject("nested_chapters"));
}