### 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
debugTestCase(
                    builder,
                    new MatchAllDocsQuery(),
                    searcher,
                    (InternalFilters filters, Class<? extends Aggregator> impl, Map<String, Map<String, Object>> debug) -> {
                        assertThat(filters.getBuckets(), hasSize(1));
                        assertThat(filters.getBucketByKey("q1").getDocCount(), equalTo(1L));
                        assertThat(impl, equalTo(FilterByFilterAggregator.class));
                        assertMap(
                            debug,
                            matchesMap().entry(
                                "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", "test:\"will find me\"")
                                                .entry("segments_counted_in_constant_time", segmentsCountedInConstantTime)
                                        )
                                    )
                            )
                        );
                    },
                    ft
                );
            }
        });