### 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
return new QueryToFilterAdapter(searcher, key, query);
    }
    private final IndexSearcher searcher;
    private final String key;
    private final Query query;
    /**
     * The weight for the query or {@code null} if we haven't built it. Use
     * {@link #weight()} to build it when needed.
     */
    private Weight weight;
    protected int segmentsCountedInConstantTime;
    QueryToFilterAdapter(IndexSearcher searcher, String key, Query query) {
        this.searcher = searcher;
        this.key = key;
        this.query = query;
    }
    /**
     * The query we're adapting.
     * <p>
     * Subclasses should use this to fetch the query when making query
     * specific optimizations.
     */
    Query query() {
        return query;
    }
    /**
     * Is this an inefficient union of the top level query with the filter?
     * If the top level query if complex we can't efficiently merge it with
     * the filter. If we can't do that it is likely faster to just run the
     * "native" aggregation implementation rather than go filter by filter.