### 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
Document document = new Document();
        for (int i = 0; i < numDocs; i++) {
            if (frequently()) {
                
                indexWriter.commit();
            }
            int value = randomInt(maxTerm - 1);
            expectedBucketCount[value] += 1;
            document.add(new Field("field", new BytesRef(Integer.toString(value)), KeywordFieldMapper.Defaults.FIELD_TYPE));
            indexWriter.addDocument(document);
            document.clear();
        }
        indexWriter.close();
        DirectoryReader indexReader = DirectoryReader.open(directory);
        IndexSearcher indexSearcher = newIndexSearcher(indexReader);
        try {
            int numFilters = randomIntBetween(1, 10);
            QueryBuilder[] filters = new QueryBuilder[numFilters];
            int[] filterTerms = new int[numFilters];
            int expectedOtherCount = numDocs;
            Set<Integer> filterSet = new HashSet<>();
            for (int i = 0; i < filters.length; i++) {
                int value = randomInt(maxTerm - 1);
                filters[i] = QueryBuilders.termQuery("field", Integer.toString(value));
                filterTerms[i] = value;
                if (filterSet.contains(value) == false) {
                    expectedOtherCount -= expectedBucketCount[value];
                    filterSet.add(value);
                }
            }