### 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
final DocCountProvider docCount;
        private long count;
        Counter(DocCountProvider docCount) {
            this.docCount = docCount;
        }
        public long readAndReset(LeafReaderContext ctx) throws IOException {
            long result = count;
            count = 0;
            docCount.setLeafReaderContext(ctx);
            return result;
        }
        @Override
        public void collect(int doc) throws IOException {
            count += docCount.getDocCount(doc);
        }
        @Override
        public void setScorer(Scorable scorer) throws IOException {}
    }
}