### 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
matchesMap().entry("segments_with_doc_count_field", 0)
                            .entry("segments_with_deleted_docs", 0)
                            .entry("segments_collected", greaterThanOrEqualTo(1))
                            .entry("segments_counted", 0)
                            .entry("filters", filtersMatcher)
                    ).entry("test.s", matchesMap()).entry("test.m", matchesMap())
                );
            },
            dateFt,
            intFt
        );
    }
    public void testSubAggsManyDocs() throws IOException {
        MappedFieldType dateFt = new DateFieldMapper.DateFieldType(
            "test",
            true,
            false,
            false,
            DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER,
            Resolution.MILLISECONDS,
            null,
            null,
            Collections.emptyMap()
        );
        MappedFieldType intFt = new NumberFieldMapper.NumberFieldType("int", NumberType.INTEGER);
        AggregationBuilder builder = new FiltersAggregationBuilder(
            "test",
            new KeyedFilter("q1", new RangeQueryBuilder("test").from("2010-01-01").to("2010-03-01").includeUpper(false)),
            new KeyedFilter("q2", new RangeQueryBuilder("test").from("2020-01-01").to("2020-03-01").includeUpper(false))
        ).subAggregation(new MaxAggregationBuilder("m").field("int")).subAggregation(new SumAggregationBuilder("s").field("int"));