Implemented cost function

Signed-off-by: expani <[email protected]>

Author: expani

server/src/main/java/org/opensearch/search/aggregations/metrics/CardinalityAggregator.java

@@ -458,8 +458,7 @@ public int nextDoc() throws IOException {
 
         @Override
         public long cost() {
-            // don't expect this to be called based on its usage in DefaultBulkScorer
-            throw new UnsupportedOperationException();
+            return queue.top() == null ? 0 : queue.top().approximation.cost();
         }
     }