### 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
* the {@link #segmentsCounted} counter because those counters are to
     * signal which operation we were allowed to perform. The filters
     * themselves will have debugging counters measuring if they could
     * perform the count from metadata or had to fall back.
     */
    private int segmentsCounted;
    /**
     * Build the aggregation. Private to force callers to go through the
     * {@link AdapterBuilder} which centralizes the logic to decide if this
     * aggregator would be faster than the native implementation.
     */
    private FilterByFilterAggregator(
        String name,
        AggregatorFactories factories,
        List<QueryToFilterAdapter> filters,
        boolean keyed,
        boolean keyedBucket,
        AggregationContext aggCtx,
        Aggregator parent,
        CardinalityUpperBound cardinality,
        Map<String, Object> metadata
    ) throws IOException {
        super(name, factories, filters, keyed, keyedBucket, null, aggCtx, parent, cardinality, metadata);
    }
    /**
     * Instead of returning a {@link LeafBucketCollector} we do the
     * collection ourselves by running the filters directly. This is safe
     * because we only use this aggregator if there isn't a {@code parent}
     * which would change how we collect buckets and because we take the
     * top level query into account when building the filters.