### 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
builder,
            new MatchAllDocsQuery(),
            iw -> iw.addDocuments(docs),
            (InternalFilters filters, Class<? extends Aggregator> impl, Map<String, Map<String, Object>> debug) -> {
                assertThat(filters.getBuckets(), hasSize(buckets.size()));
                InternalFilters.InternalBucket b = filters.getBucketByKey("2010-01-01 to 2010-01-31");
                assertThat(b.getDocCount(), equalTo(3334L));
                Max max = b.getAggregations().get("m");
                assertThat(max.value(), equalTo(9999.0));
                Sum sum = b.getAggregations().get("s");
                assertThat(sum.value(), equalTo(16668333.0));
                b = filters.getBucketByKey("2019-12-10 to 2020-01-09");
                assertThat(b.getDocCount(), equalTo(6666L));
                max = b.getAggregations().get("m");
                assertThat(max.value(), equalTo(9998.0));
                sum = b.getAggregations().get("s");
                assertThat(sum.value(), equalTo(33326667.0));
                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", greaterThanOrEqualTo(1))
                            .entry("segments_counted", 0)
                            .entry("filters", hasSize(buckets.size()))
                    ).entry("test.s", matchesMap()).entry("test.m", matchesMap())
                );
            },