### 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
FiltersAggregationBuilder builder = new FiltersAggregationBuilder("test", filters);
            builder.otherBucket(true);
            builder.otherBucketKey("other");
            final InternalFilters response = searchAndReduce(indexSearcher, new AggTestConfig(builder, new KeywordFieldType("field")));
            List<InternalFilters.InternalBucket> buckets = response.getBuckets();
            assertEquals(buckets.size(), filters.length + 1);
            for (InternalFilters.InternalBucket bucket : buckets) {
                if ("other".equals(bucket.getKey())) {
                    assertEquals(bucket.getDocCount(), expectedOtherCount);
                } else {
                    int index = Integer.parseInt(bucket.getKey());
                    assertEquals(bucket.getDocCount(), (long) expectedBucketCount[filterTerms[index]]);
                }
            }
            
            assertTrue(AggregationInspectionHelper.hasValue(response));
        } finally {
            indexReader.close();
            directory.close();
        }
    }
    /**
     * Test that we perform the appropriate unwrapping to merge queries.
     */
    public void testMergingQueries() throws IOException {
        DateFieldMapper.DateFieldType ft = new DateFieldMapper.DateFieldType("test");
        Query topLevelQuery = ft.rangeQuery("2020-01-01", "2020-02-01", true, true, null, null, null, mock(SearchExecutionContext.class));
        FiltersAggregationBuilder builder = new FiltersAggregationBuilder(
            "t",