### 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/FilterByFilterAggregator.java
*/
    @Override
    protected LeafBucketCollector getLeafCollector(AggregationExecutionContext aggCtx, LeafBucketCollector sub) throws IOException {
        assert scoreMode().needsScores() == false;
        if (filters().size() == 0) {
            return LeafBucketCollector.NO_OP_COLLECTOR;
        }
        Bits live = aggCtx.getLeafReaderContext().reader().getLiveDocs();
        if (live != null) {
            segmentsWithDeletedDocs++;
        }
        if (false == docCountProvider.alwaysOne()) {
            segmentsWithDocCountField++;
        }
        if (subAggregators.length == 0) {
            
            /*
             * Without sub.isNoop we always end up in the `collectXXX` modes even if
             * the sub-aggregators opt out of traditional collection.
             */
            segmentsCounted++;
            collectCount(aggCtx.getLeafReaderContext(), live);
        } else {
            segmentsCollected++;
            collectSubs(aggCtx, live, sub);
        }
        return LeafBucketCollector.NO_OP_COLLECTOR;
    }
    /**
     * Gather a count of the number of documents that match each filter