### 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
private FilterByFilterAggregator agg;
                @Override
                public FilterByFilterAggregator apply(AggregatorFactories subAggregators) throws IOException {
                    agg = new FilterByFilterAggregator(
                        name,
                        subAggregators,
                        filters,
                        keyed,
                        keyedBucket,
                        aggCtx,
                        parent,
                        cardinality,
                        metadata
                    );
                    return agg;
                }
            }
            AdapterBuild adapterBuild = new AdapterBuild();
            T result = adapt(adapterBuild);
            if (adapterBuild.agg.scoreMode().needsScores()) {
                /*
                 * Filter by filter won't produce the correct results if the
                 * sub-aggregators need scores because we're not careful with how
                 * we merge filters. Right now we have to build the whole
                 * aggregation in order to know if it'll need scores or not.
                 * This means we'll build the *sub-aggs* too. Oh well.
                 */