### 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
"f",
                randomFrom(NumberFieldMapper.NumberType.values()),
                true,
                false,
                true,
                true,
                null,
                Map.of(),
                null,
                false,
                null,
                null
            )
        );
    }
    public void testDocValuesFieldExistsForKeyword() throws IOException {
        KeywordFieldMapper.KeywordFieldType ft = new KeywordFieldMapper.KeywordFieldType("f", true, true, Map.of());
        docValuesFieldExistsTestCase(new ExistsQueryBuilder("f"), ft, true, i -> {
            BytesRef text = new BytesRef(randomAlphaOfLength(5));
            return List.of(new Field("f", text, KeywordFieldMapper.Defaults.FIELD_TYPE));
        });
    }
    public void testDocValuesFieldExistsForKeywordWithoutData() throws IOException {
        docValuesFieldExistsNoDataTestCase(new KeywordFieldMapper.KeywordFieldType("f", true, true, Map.of()));
    }
    private void docValuesFieldExistsTestCase(
        QueryBuilder exists,
        MappedFieldType fieldType,
        boolean countsResultsInConstantTime,
        IntFunction<Iterable<? extends IndexableField>> buildDocWithField