### 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/QueryToFilterAdapter.java
/*
         * weight().count will return the count of matches for ctx if it can do
         * so in constant time, otherwise -1. The Weight is responsible for
         * all of the cases where it can't return an accurate count *except*
         * the doc_count field. That thing is ours, not Lucene's.
         *
         * For example, TermQuery will return -1 if there are deleted docs,
         * otherwise it'll return number of documents with the term from the
         * term statistics. MatchAllDocs will call `ctx.reader().numdocs()`
         * to get the number of live docs.
         */
        if (counter.docCount.alwaysOne()) {
            int count = weight().count(ctx);
            if (count != -1) {
                segmentsCountedInConstantTime++;
                return count;
            }
        }
        BulkScorer scorer = weight().bulkScorer(ctx);
        if (scorer == null) {
            
            return 0;
        }
        scorer.score(counter, live);
        return counter.readAndReset(ctx);
    }
    /**
     * Collect all documents that match this filter in this leaf.
     */
    void collect(LeafReaderContext ctx, LeafCollector collector, Bits live) throws IOException {