### 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
public static final ParseField FILTERS_FIELD = new ParseField("filters");
    public static final ParseField OTHER_BUCKET_FIELD = new ParseField("other_bucket");
    public static final ParseField OTHER_BUCKET_KEY_FIELD = new ParseField("other_bucket_key");
    public static final ParseField KEYED_FIELD = new ParseField("keyed");
    public static class KeyedFilter implements Writeable, ToXContentFragment {
        private final String key;
        private final QueryBuilder filter;
        public KeyedFilter(String key, QueryBuilder filter) {
            if (key == null) {
                throw new IllegalArgumentException("[key] must not be null");
            }
            if (filter == null) {
                throw new IllegalArgumentException("[filter] must not be null");
            }
            this.key = key;
            this.filter = filter;