### 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/FilterByFilterAggregator.java
private final String name;
        private final List<QueryToFilterAdapter> filters = new ArrayList<>();
        private final boolean keyed;
        private final boolean keyedBucket;
        private final AggregationContext aggCtx;
        private final Aggregator parent;
        private final CardinalityUpperBound cardinality;
        private final Map<String, Object> metadata;
        private final Query rewrittenTopLevelQuery;
        private boolean valid = true;
        public AdapterBuilder(
            String name,
            boolean keyed,
            boolean keyedBucket,
            String otherBucketKey,
            AggregationContext aggCtx,
            Aggregator parent,
            CardinalityUpperBound cardinality,
            Map<String, Object> metadata
        ) throws IOException {
            this.name = name;
            this.keyed = keyed;
            this.keyedBucket = keyedBucket;
            this.aggCtx = aggCtx;
            this.parent = parent;
            this.cardinality = cardinality;
            this.metadata = metadata;
            this.rewrittenTopLevelQuery = aggCtx.searcher().rewrite(aggCtx.query());
            this.valid = parent == null && otherBucketKey == null && aggCtx.isInSortOrderExecutionRequired() == false;
        }