### 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
if (obj == null) {
                return false;
            }
            if (getClass() != obj.getClass()) {
                return false;
            }
            KeyedFilter other = (KeyedFilter) obj;
            return Objects.equals(key, other.key) && Objects.equals(filter, other.filter);
        }
    }
    /**
     * Build an {@link Aggregator} for a {@code filters} aggregation. If there
     * isn't a parent, there aren't children, and we don't collect "other"
     * buckets then this will a faster {@link FilterByFilterAggregator} aggregator.
     * Otherwise it'll fall back to a slower aggregator that is
     * {@link Compatible} with parent, children, and "other" buckets.
     */
    public static FiltersAggregator build(
        String name,
        AggregatorFactories factories,
        List<QueryToFilterAdapter> filters,
        boolean keyed,
        String otherBucketKey,
        boolean keyedBucket,
        AggregationContext context,
        Aggregator parent,
        CardinalityUpperBound cardinality,
        Map<String, Object> metadata
    ) throws IOException {
        FilterByFilterAggregator.AdapterBuilder<FilterByFilterAggregator> filterByFilterBuilder =