### 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
return filters;
    }
    @Override
    public InternalAggregation[] buildAggregations(long[] owningBucketOrds) throws IOException {
        return buildAggregationsForFixedBucketCount(
            owningBucketOrds,
            filters.size() + (otherBucketKey == null ? 0 : 1),
            (offsetInOwningOrd, docCount, subAggregationResults) -> {
                if (offsetInOwningOrd < filters.size()) {
                    return new InternalFilters.InternalBucket(
                        filters.get(offsetInOwningOrd).key().toString(),
                        docCount,
                        subAggregationResults,
                        keyed,
                        keyedBucket
                    );
                }
                return new InternalFilters.InternalBucket(otherBucketKey, docCount, subAggregationResults, keyed, keyedBucket);
            },
            buckets -> new InternalFilters(name, buckets, keyed, keyedBucket, metadata())
        );
    }
    @Override
    public InternalAggregation buildEmptyAggregation() {
        InternalAggregations subAggs = buildEmptySubAggregations();
        List<InternalFilters.InternalBucket> buckets = new ArrayList<>(filters.size() + (otherBucketKey == null ? 0 : 1));
        for (QueryToFilterAdapter filter : filters) {
            InternalFilters.InternalBucket bucket = new InternalFilters.InternalBucket(
                filter.key().toString(),
                0,