### 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/main/java/org/elasticsearch/search/aggregations/bucket/filter/FiltersAggregator.java
otherBucketKey,
            context,
            parent,
            cardinality,
            metadata
        );
    }
    private final List<QueryToFilterAdapter> filters;
    private final boolean keyed;
    private final boolean keyedBucket;
    protected final String otherBucketKey;
    FiltersAggregator(
        String name,
        AggregatorFactories factories,
        List<QueryToFilterAdapter> filters,
        boolean keyed,
        boolean keyedBucket,
        String otherBucketKey,
        AggregationContext aggCtx,
        Aggregator parent,
        CardinalityUpperBound cardinality,
        Map<String, Object> metadata
    ) throws IOException {
        super(name, factories, aggCtx, parent, cardinality.multiply(filters.size() + (otherBucketKey == null ? 0 : 1)), metadata);
        this.filters = List.copyOf(filters);
        this.keyed = keyed;
        this.keyedBucket = keyedBucket;
        this.otherBucketKey = otherBucketKey;
    }
    List<QueryToFilterAdapter> filters() {