Fix max-int limit for number of points reduced in geo_centroid (#56370)

A bug in InternalGeoCentroid#reduce existed that summed up
the aggregation's long-valued counts into a local integer variable.
Since it is definitely possible to reduce more than Integer.MAX points,
this change simply updates that variable to be a long-valued number.

Closes #55992.

server/src/main/java/org/elasticsearch/search/aggregations/metrics/InternalGeoCentroid.java

@@ -115,7 +115,7 @@ public class InternalGeoCentroid extends InternalAggregation implements GeoCentr
     public InternalGeoCentroid reduce(List<InternalAggregation> aggregations, ReduceContext reduceContext) {
         double lonSum = Double.NaN;
         double latSum = Double.NaN;
-        int totalCount = 0;
+        long totalCount = 0;
         for (InternalAggregation aggregation : aggregations) {
             InternalGeoCentroid centroidAgg = (InternalGeoCentroid) aggregation;
             if (centroidAgg.count > 0) {

server/src/test/java/org/elasticsearch/search/aggregations/metrics/InternalGeoCentroidTests.java

@@ -29,6 +29,8 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
+import static org.hamcrest.Matchers.equalTo;
+
 public class InternalGeoCentroidTests extends InternalAggregationTestCase<InternalGeoCentroid> {
 
     @Override
@@ -52,7 +54,7 @@ public class InternalGeoCentroidTests extends InternalAggregationTestCase<Intern
     protected void assertReduced(InternalGeoCentroid reduced, List<InternalGeoCentroid> inputs) {
         double lonSum = 0;
         double latSum = 0;
-        int totalCount = 0;
+        long totalCount = 0;
         for (InternalGeoCentroid input : inputs) {
             if (input.count() > 0) {
                 lonSum += (input.count() * input.centroid().getLon());
@@ -67,6 +69,14 @@ public class InternalGeoCentroidTests extends InternalAggregationTestCase<Intern
         assertEquals(totalCount, reduced.count());
     }
 
+    public void testReduceMaxCount() {
+        InternalGeoCentroid maxValueGeoCentroid = new InternalGeoCentroid("agg", new GeoPoint(10, 0),
+            Long.MAX_VALUE, Collections.emptyMap());
+        InternalGeoCentroid reducedGeoCentroid = maxValueGeoCentroid
+            .reduce(Collections.singletonList(maxValueGeoCentroid), null);
+        assertThat(reducedGeoCentroid.count(), equalTo(Long.MAX_VALUE));
+    }
+
     @Override
     protected void assertFromXContent(InternalGeoCentroid aggregation, ParsedAggregation parsedAggregation) {
         assertTrue(parsedAggregation instanceof ParsedGeoCentroid);