### 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/QueryToFilterAdapter.java
*/
    public boolean isInefficientUnion() {
        return false;
    }
    /**
     * Key for this filter.
     */
    public final String key() {
        return key;
    }
    /**
     * Searcher that this filter is targeting.
     */
    protected final IndexSearcher searcher() {
        return searcher;
    }
    /**
     * Make a filter that matches this filter and the provided query.
     * <p>
     * Note: This method rewrites the query against the {@link IndexSearcher}.
     */
    QueryToFilterAdapter union(Query extraQuery) throws IOException {
        /*
         * Wrapping with a ConstantScoreQuery enables a few more rewrite
         * rules as of Lucene 9.2.
         * It'd be *wonderful* if Lucene could do fancy optimizations
         * when merging queries like combining ranges but it doesn't at
         * the moment. Admittedly, we have a much more limited problem.
         * We don't care about score here at all. We know which queries
         * it's worth spending time to optimize because we know which aggs