Add cluster stats rolling version upgrade ITs.

Signed-off-by: Swetha Guptha <gupthasg@amazon.com>

qa/rolling-upgrade/src/test/java/org/opensearch/upgrades/ClusterStatsIT.java

@@ -0,0 +1,78 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.upgrades;
+
+import org.opensearch.Version;
+import org.opensearch.client.Request;
+import org.opensearch.client.Response;
+import org.opensearch.cluster.metadata.IndexMetadata;
+import org.opensearch.common.settings.Settings;
+
+import java.io.IOException;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+
+public class ClusterStatsIT extends AbstractRollingTestCase {
+
+    public void testClusterStats() throws IOException {
+        if (CLUSTER_TYPE == ClusterType.OLD) {
+            final String indexName = "test-index";
+            final int shardCount = 3;
+            final int replicaCount = 1;
+            Settings settings = Settings.builder()
+                .put(IndexMetadata.INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), shardCount)
+                .put(IndexMetadata.INDEX_NUMBER_OF_REPLICAS_SETTING.getKey(), replicaCount)
+                .build();
+            createIndex(indexName, settings);
+            Request waitForStatus = new Request("GET", "/_cluster/health/" + indexName);
+            waitForStatus.addParameter("wait_for_status", "green");
+            client().performRequest(waitForStatus);
+        }
+
+        Response response = client().performRequest(new Request("GET", "/_cluster/stats"));
+        validateClusterStatsWithFilterResponse(response, List.of("os", "process", "jvm", "fs", "plugins", "ingest", "network_types", "discovery_types", "packaging_types"),
+            List.of("shards", "docs", "store", "fielddata", "query_cache", "completion", "segments", "analysis", "mappings"));
+        if (AbstractRollingTestCase.UPGRADE_FROM_VERSION.onOrAfter(Version.V_3_0_0) || (
+            CLUSTER_TYPE == ClusterType.UPGRADED && Version.CURRENT.onOrAfter(Version.V_3_0_0))) {
+            response = client().performRequest(new Request("GET", "/_cluster/stats/os/nodes/_all"));
+            validateClusterStatsWithFilterResponse(response, List.of("os"), Collections.emptyList());
+            response = client().performRequest(new Request("GET", "/_cluster/stats/indices/nodes/_all"));
+            validateClusterStatsWithFilterResponse(response, Collections.emptyList(), List.of("shards"));
+            response = client().performRequest(new Request("GET", "/_cluster/stats/os,indices/nodes/_all"));
+            validateClusterStatsWithFilterResponse(response, List.of("os"), List.of("shards"));
+        }
+    }
+
+    private void validateClusterStatsWithFilterResponse(Response response, List<String> nodesStatsMetrics, List<String> indicesStatsMetrics) throws IOException {
+        assertEquals(200, response.getStatusLine().getStatusCode());
+        Map<String, Object> entity = entityAsMap(response);
+        if (nodesStatsMetrics != null && !nodesStatsMetrics.isEmpty()) {
+            assertTrue(entity.containsKey("nodes"));
+            Map<?, ?> nodesStats = ( Map<?, ?>) entity.get("nodes");
+            for (String metric : nodesStatsMetrics) {
+                assertTrue(nodesStats.containsKey(metric));
+            }
+        }
+        if (indicesStatsMetrics != null && !indicesStatsMetrics.isEmpty()) {
+            assertTrue(entity.containsKey("indices"));
+            Map<?, ?> indicesStats = ( Map<?, ?>) entity.get("indices");
+            for (String metric : indicesStatsMetrics) {
+                assertTrue(indicesStats.containsKey(metric));
+                if (metric.equals("shards")) {
+                    Map<?, ?> shards = (Map<?, ?>) indicesStats.get("shards");
+                    assertTrue(shards.containsKey("total"));
+                    assertEquals(6, (int) shards.get("total"));
+                    assertTrue(shards.containsKey("primaries"));
+                    assertEquals(3, (int) shards.get("primaries"));
+                }
+            }
+        }
+    }
+}