### 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/main/java/org/elasticsearch/search/aggregations/bucket/filter/FiltersAggregator.java
protected LeafBucketCollector getLeafCollector(AggregationExecutionContext aggCtx, LeafBucketCollector sub) throws IOException {
            IntPredicate[] docFilters = new IntPredicate[filters().size()];
            for (int filterOrd = 0; filterOrd < filters().size(); filterOrd++) {
                docFilters[filterOrd] = filters().get(filterOrd).matchingDocIds(aggCtx.getLeafReaderContext());
            }
            return new LeafBucketCollectorBase(sub, null) {
                @Override
                public void collect(int doc, long bucket) throws IOException {
                    boolean matched = false;
                    for (int i = 0; i < docFilters.length; i++) {
                        if (docFilters[i].test(doc)) {
                            collectBucket(sub, doc, bucketOrd(bucket, i));
                            matched = true;
                        }
                    }
                    if (otherBucketKey != null && false == matched) {
                        collectBucket(sub, doc, bucketOrd(bucket, docFilters.length));
                    }
                }
            };
        }
        final long bucketOrd(long owningBucketOrdinal, int filterOrd) {
            return owningBucketOrdinal * totalNumKeys + filterOrd;
        }
    }
    /**
     * Counts collected documents, delegating to {@link DocCountProvider} for
     * how many documents each search hit is "worth".
     */
    static class Counter implements LeafCollector {