Make hits.total an object in the search response (#35849)

This commit changes the format of the `hits.total` in the search response to be an object with
a `value` and a `relation`. The `value` indicates the number of hits that match the query and the
`relation` indicates whether the number is accurate (in which case the relation is equals to `eq`)
or a lower bound of the total (in which case it is equals to `gte`).
This change also adds a parameter called `rest_total_hits_as_int` that can be used in the
search APIs to opt out from this change (retrieve the total hits as a number in the rest response).
Note that currently all search responses are accurate (`track_total_hits: true`) or they don't contain
`hits.total` (`track_total_hits: true`). We'll add a way to get a lower bound of the total hits in a
follow up (to allow numbers to be passed to `track_total_hits`).

Relates #33028

Author: jimczi

client/client-benchmark-noop-api-plugin/src/main/java/org/elasticsearch/plugin/noop/action/search/TransportNoopSearchAction.java

@@ -18,6 +18,7 @@
  */
 package org.elasticsearch.plugin.noop.action.search;
 
+import org.apache.lucene.search.TotalHits;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.search.SearchRequest;
 import org.elasticsearch.action.search.SearchResponse;
@@ -47,10 +48,10 @@ public TransportNoopSearchAction(TransportService transportService, ActionFilter
     protected void doExecute(Task task, SearchRequest request, ActionListener<SearchResponse> listener) {
         listener.onResponse(new SearchResponse(new InternalSearchResponse(
             new SearchHits(
-                new SearchHit[0], 0L, 0.0f),
+                new SearchHit[0], new TotalHits(0L, TotalHits.Relation.EQUAL_TO), 0.0f),
             new InternalAggregations(Collections.emptyList()),
             new Suggest(Collections.emptyList()),
-            new SearchProfileShardResults(Collections.emptyMap()), false, false, 1), "", 1, 1, 0, 0, ShardSearchFailure.EMPTY_ARRAY,
-                SearchResponse.Clusters.EMPTY));
+            new SearchProfileShardResults(Collections.emptyMap()), false, false, 1),
+            "", 1, 1, 0, 0, ShardSearchFailure.EMPTY_ARRAY, SearchResponse.Clusters.EMPTY));
     }
 }

client/rest-high-level/src/test/java/org/elasticsearch/client/CCRIT.java

@@ -104,7 +104,7 @@ public void testIndexFollowing() throws Exception {
 
         SearchRequest leaderSearchRequest = new SearchRequest("leader");
         SearchResponse leaderSearchResponse = highLevelClient().search(leaderSearchRequest, RequestOptions.DEFAULT);
-        assertThat(leaderSearchResponse.getHits().getTotalHits(), equalTo(1L));
+        assertThat(leaderSearchResponse.getHits().getTotalHits().value, equalTo(1L));
 
         assertBusy(() -> {
             CcrStatsRequest ccrStatsRequest = new CcrStatsRequest();
@@ -118,7 +118,7 @@ public void testIndexFollowing() throws Exception {
 
             SearchRequest followerSearchRequest = new SearchRequest("follower");
             SearchResponse followerSearchResponse = highLevelClient().search(followerSearchRequest, RequestOptions.DEFAULT);
-            assertThat(followerSearchResponse.getHits().getTotalHits(), equalTo(1L));
+            assertThat(followerSearchResponse.getHits().getTotalHits().value, equalTo(1L));
         });
 
         PauseFollowRequest pauseFollowRequest = new PauseFollowRequest("follower");
@@ -143,7 +143,7 @@ public void testIndexFollowing() throws Exception {
 
             SearchRequest followerSearchRequest = new SearchRequest("follower");
             SearchResponse followerSearchResponse = highLevelClient().search(followerSearchRequest, RequestOptions.DEFAULT);
-            assertThat(followerSearchResponse.getHits().getTotalHits(), equalTo(2L));
+            assertThat(followerSearchResponse.getHits().getTotalHits().value, equalTo(2L));
         });
 
         // Need to pause prior to unfollowing it:

client/rest-high-level/src/test/java/org/elasticsearch/client/CrudIT.java

@@ -882,7 +882,7 @@ public void testDeleteByQuery() throws Exception {
             assertEquals(0, bulkResponse.getSearchFailures().size());
             assertEquals(
                 2,
-                highLevelClient().search(new SearchRequest(sourceIndex), RequestOptions.DEFAULT).getHits().totalHits
+                highLevelClient().search(new SearchRequest(sourceIndex), RequestOptions.DEFAULT).getHits().getTotalHits().value
             );
         }
         {

client/rest-high-level/src/test/java/org/elasticsearch/client/RestHighLevelClientTests.java

@@ -174,7 +174,7 @@ public void testSearchScroll() throws IOException {
         SearchResponse searchResponse = restHighLevelClient.scroll(
                 new SearchScrollRequest(randomAlphaOfLengthBetween(5, 10)), RequestOptions.DEFAULT);
         assertEquals(mockSearchResponse.getScrollId(), searchResponse.getScrollId());
-        assertEquals(0, searchResponse.getHits().totalHits);
+        assertEquals(0, searchResponse.getHits().getTotalHits().value);
         assertEquals(5, searchResponse.getTotalShards());
         assertEquals(5, searchResponse.getSuccessfulShards());
         assertEquals(100, searchResponse.getTook().getMillis());

client/rest-high-level/src/test/java/org/elasticsearch/client/RollupIT.java

@@ -189,7 +189,7 @@ public void testPutStartAndGetRollupJob() throws Exception {
         assertBusy(() -> {
             SearchResponse searchResponse = highLevelClient().search(new SearchRequest(rollupIndex), RequestOptions.DEFAULT);
             assertEquals(0, searchResponse.getFailedShards());
-            assertEquals(1L, searchResponse.getHits().getTotalHits());
+            assertEquals(1L, searchResponse.getHits().getTotalHits().value);
 
             SearchHit searchHit = searchResponse.getHits().getAt(0);
             Map<String, Object> source = searchHit.getSourceAsMap();