### 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
BulkScorer scorer = weight().bulkScorer(ctx);
        if (scorer == null) {
            
            return;
        }
        scorer.score(collector, live);
    }
    /**
     * Collect profiling information for this filter. Rhymes with
     * {@link Aggregator#collectDebugInfo(BiConsumer)}.
     * <p>
     * Well behaved implementations will always call the superclass
     * implementation just in case it has something interesting. They will
     * also only add objects which can be serialized with
     * {@link StreamOutput#writeGenericValue(Object)} and
     * {@link XContentBuilder#value(Object)}. And they'll have an integration
     * test.
     */
    void collectDebugInfo(BiConsumer<String, Object> add) {
        add.accept("query", query.toString());
        add.accept("segments_counted_in_constant_time", segmentsCountedInConstantTime);
    }
    private Weight weight() throws IOException {
        if (weight == null) {
            weight = searcher().createWeight(query, ScoreMode.COMPLETE_NO_SCORES, 1.0f);
        }
        return weight;
    }
}