### 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
private final int totalNumKeys;
        Compatible(
            String name,
            AggregatorFactories factories,
            List<QueryToFilterAdapter> filters,
            boolean keyed,
            boolean keyedBucket,
            String otherBucketKey,
            AggregationContext context,
            Aggregator parent,
            CardinalityUpperBound cardinality,
            Map<String, Object> metadata
        ) throws IOException {
            super(name, factories, filters, keyed, keyedBucket, otherBucketKey, context, parent, cardinality, metadata);
            if (otherBucketKey == null) {
                this.totalNumKeys = filters.size();
            } else {
                this.totalNumKeys = filters.size() + 1;
            }
        }
        @Override