Fix terms aggregation ordering after the final reduce (#62732)

This commit ensures that the final order of the terms aggregations
is registered correctly after the final reduce.
This bug was introduced in #62028 which is not released yet so this PR is marked
as a non-issue.
This issue was discovered when running a terms aggregation under an auto-date
histogram. In such a case, the auto-date histogram may run multiple final reduce
to merge buckets together. This change makes sure that running multiple final reduces
doesn't create duplicates but it doesn't fix the fact that the final reduce may prune
the list of terms prematurely. This other bug is tracked separately in #62731.

Author: jimczi

server/src/main/java/org/elasticsearch/search/aggregations/bucket/terms/InternalTerms.java

@@ -426,7 +426,7 @@ public InternalAggregation reduce(List<InternalAggregation> aggregations, Reduce
         } else {
             docCountError = aggregations.size() == 1 ? 0 : sumDocCountError;
         }
-        return create(name, Arrays.asList(list), thisReduceOrder, docCountError, otherDocCount);
+        return create(name, Arrays.asList(list), reduceContext.isFinalReduce() ? order : thisReduceOrder, docCountError, otherDocCount);
     }
 
     @Override

server/src/test/java/org/elasticsearch/search/aggregations/bucket/terms/InternalTermsTestCase.java

@@ -31,6 +31,8 @@
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
+import static org.hamcrest.Matchers.equalTo;
+
 public abstract class InternalTermsTestCase extends InternalMultiBucketAggregationTestCase<InternalTerms<?, ?>> {
 
     private boolean showDocCount;
@@ -88,6 +90,10 @@ protected void assertReduced(InternalTerms<?, ?> reduced, List<InternalTerms<?,
         final long expectedTotalDocCount = inputs.stream().map(Terms::getBuckets)
                 .flatMap(List::stream).mapToLong(Terms.Bucket::getDocCount).sum();
         assertEquals(expectedTotalDocCount, reducedTotalDocCount);
+        for (InternalTerms<?, ?> terms : inputs) {
+            assertThat(reduced.reduceOrder, equalTo(terms.order));
+            assertThat(reduced.order, equalTo(terms.order));
+        }
     }
 
     private static Map<Object, Long> toCounts(Stream<? extends Terms.Bucket> buckets) {