### 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
}
        /**
         * Read from a stream.
         */
        public KeyedFilter(StreamInput in) throws IOException {
            key = in.readString();
            filter = in.readNamedWriteable(QueryBuilder.class);
        }
        @Override
        public void writeTo(StreamOutput out) throws IOException {
            out.writeString(key);
            out.writeNamedWriteable(filter);
        }
        public String key() {
            return key;
        }
        public QueryBuilder filter() {
            return filter;
        }
        @Override
        public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
            builder.field(key, filter);
            return builder;
        }
        @Override
        public int hashCode() {
            return Objects.hash(key, filter);
        }
        @Override
        public boolean equals(Object obj) {