### 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/test/java/org/elasticsearch/search/aggregations/bucket/filter/FiltersAggregatorTests.java
* bad implementations of filter-by-filter aggregation to fail with
         * assertion errors while executing.
         */
        Collections.shuffle(docs, random());
        debugTestCase(
            builder,
            new MatchAllDocsQuery(),
            iw -> iw.addDocuments(docs),
            (InternalFilters filters, Class<? extends Aggregator> impl, Map<String, Map<String, Object>> debug) -> {
                assertThat(filters.getBuckets(), hasSize(2));
                InternalFilters.InternalBucket b = filters.getBucketByKey("q1");
                assertThat(b.getDocCount(), equalTo(1L));
                Max max = b.getAggregations().get("m");
                assertThat(max.value(), equalTo(100.0));
                Sum sum = b.getAggregations().get("s");
                assertThat(sum.value(), equalTo(100.0));
                b = filters.getBucketByKey("q2");
                assertThat(b.getDocCount(), equalTo(2L));
                max = b.getAggregations().get("m");
                assertThat(max.value(), equalTo(10.0));
                sum = b.getAggregations().get("s");
                assertThat(sum.value(), equalTo(15.0));
                assertThat(impl, equalTo(FilterByFilterAggregator.class));
                ListMatcher filtersMatcher = matchesList().item(
                    matchesMap().entry("query", "test:[1262304000000 TO 1267401599999]").entry("segments_counted_in_constant_time", 0)
                ).item(matchesMap().entry("query", "test:[1577836800000 TO 1583020799999]").entry("segments_counted_in_constant_time", 0));
                assertMap(
                    debug,
                    matchesMap().entry(
                        "test",