### Description

Follow-up of #98360: when there is a single filter, the collector could save the overhead of the priority queue (both in `collect()` and `competitiveIterator()`), which would likely result in a speedup.
server/src/main/java/org/elasticsearch/search/aggregations/bucket/filter/FiltersAggregator.java
final DocCountProvider docCount;
        private long count;
        Counter(DocCountProvider docCount) {
            this.docCount = docCount;
        }
        public long readAndReset(LeafReaderContext ctx) throws IOException {
            long result = count;
            count = 0;
            docCount.setLeafReaderContext(ctx);
            return result;
        }
        @Override
        public void collect(int doc) throws IOException {
            count += docCount.getDocCount(doc);
        }
        @Override
        public void setScorer(Scorable scorer) throws IOException {}
    }
}