### 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
.entry("segments_collected", 0)
                            .entry("segments_counted", greaterThanOrEqualTo(1))
                            .entry(
                                "filters",
                                matchesList().item(
                                    matchesMap().entry(
                                        "query",
                                        "MatchNoDocsQuery(\"The \"range\" query was rewritten to a \"match_none\" query.\")"
                                    ).entry("segments_counted_in_constant_time", greaterThan(0))
                                )
                            )
                    )
                );
            }
        );
    }
    public void testTermFilter() throws IOException {
        KeywordFieldMapper.KeywordFieldType ft = new KeywordFieldMapper.KeywordFieldType("f", true, false, Collections.emptyMap());
        AggregationBuilder builder = new FiltersAggregationBuilder("test", new KeyedFilter("q1", new MatchQueryBuilder("f", "0")));
        CheckedConsumer<RandomIndexWriter, IOException> buildIndex = iw -> {
            for (int i = 0; i < 10; i++) {
                BytesRef bytes = new BytesRef(Integer.toString(i % 3));
                iw.addDocument(List.of(new Field("f", bytes, KeywordFieldMapper.Defaults.FIELD_TYPE)));
            }
        };
        debugTestCase(
            builder,
            new MatchAllDocsQuery(),
            buildIndex,
            (InternalFilters filters, Class<? extends Aggregator> impl, Map<String, Map<String, Object>> debug) -> {