### 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
matchesList().item(
                                            matchesMap().entry("query", "foo:bar")
                                                .entry("segments_counted_in_constant_time", searcher.getLeafContexts().size())
                                        )
                                    )
                            )
                        );
                    },
                    ft
                );
            }
        }
    }
    public void testComplexUnionDisabledFilterByFilter() throws IOException {
        MappedFieldType dft = new DateFieldMapper.DateFieldType(
            "date",
            true,
            false,
            false,
            DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER,
            Resolution.MILLISECONDS,
            null,
            null,
            Collections.emptyMap()
        );
        MappedFieldType kft = new KeywordFieldType("kwd");
        AggregationBuilder builder = new FiltersAggregationBuilder(
            "test",
            new KeyedFilter("q1", new RangeQueryBuilder("date").from("2020-01-01").to("2020-03-01").includeUpper(false))
        );