### 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
debugTestCase(builder, new TermQuery(new Term("kwd", "a")), iw -> {
            iw.addDocument(
                List.of(
                    new LongPoint("date", DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER.parseMillis("2010-01-02")),
                    new Field("kwd", new BytesRef("a"), KeywordFieldMapper.Defaults.FIELD_TYPE)
                )
            );
            iw.addDocument(
                List.of(
                    new LongPoint("date", DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER.parseMillis("2020-01-02")),
                    new Field("kwd", new BytesRef("a"), KeywordFieldMapper.Defaults.FIELD_TYPE)
                )
            );
        }, (InternalFilters filters, Class<? extends Aggregator> impl, Map<String, Map<String, Object>> debug) -> {
            assertThat(filters.getBuckets(), hasSize(1));
            assertThat(filters.getBucketByKey("q1").getDocCount(), equalTo(1L));
            assertThat(impl, equalTo(FiltersAggregator.Compatible.class));
            assertMap(
                debug,
                matchesMap().entry(
                    "test",
                    matchesMap().entry(
                        "filters",
                        matchesList().item(
                            matchesMap().entry("query", "date:[1577836800000 TO 1583020799999]")
                                .entry("segments_counted_in_constant_time", 0)
                        )
                    )
                )
            );