### 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
"test",
                        matchesMap().entry("segments_with_doc_count_field", 0)
                            .entry("segments_with_deleted_docs", 0)
                            .entry("segments_collected", 0)
                            .entry("segments_counted", greaterThanOrEqualTo(1))
                            .entry(
                                "filters",
                                matchesList().item(
                                    matchesMap().entry(
                                        "query",
                                        "MergedPointRange[+test:[1262390400000 TO 1262390405000] +test:[1262390400000 TO 1262390409000]]"
                                    ).entry("segments_counted_in_constant_time", greaterThanOrEqualTo(1))
                                )
                            )
                    )
                );
            },
            ft,
            fnft
        );
    }
    public void testSubAggs() throws IOException {
        MappedFieldType dateFt = new DateFieldMapper.DateFieldType(
            "test",
            true,
            false,
            false,
            DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER,
            Resolution.MILLISECONDS,
            null,