[Test] Ensure latest cluster info is used in tests (elastic#116908)

DesiredBalanceMetrics#updateMetrics is called on reroute which is
frequent but not guaranteed. It can also take some time to complete.
This PR adds an explicit reroute to ensure the latest cluster info is
picked up.

Resolves: elastic#116870

Author: ywangd

muted-tests.yml

@@ -240,9 +240,6 @@ tests:
 - class: org.elasticsearch.index.mapper.vectors.MultiDenseVectorScriptDocValuesTests
   method: testFloatGetVectorValueAndGetMagnitude
   issue: https://github.com/elastic/elasticsearch/issues/116863
-- class: org.elasticsearch.cluster.routing.allocation.allocator.DesiredBalanceReconcilerMetricsIT
-  method: testDesiredBalanceMetrics
-  issue: https://github.com/elastic/elasticsearch/issues/116870
 - class: org.elasticsearch.xpack.inference.InferenceRestIT
   method: test {p0=inference/30_semantic_text_inference/Calculates embeddings using the default ELSER 2 endpoint}
   issue: https://github.com/elastic/elasticsearch/issues/116542

server/src/internalClusterTest/java/org/elasticsearch/cluster/routing/allocation/allocator/DesiredBalanceReconcilerMetricsIT.java

@@ -9,6 +9,7 @@
 
 package org.elasticsearch.cluster.routing.allocation.allocator;
 
+import org.elasticsearch.action.admin.cluster.reroute.ClusterRerouteUtils;
 import org.elasticsearch.cluster.ClusterInfoService;
 import org.elasticsearch.cluster.ClusterInfoServiceUtils;
 import org.elasticsearch.cluster.InternalClusterInfoService;
@@ -68,6 +69,7 @@ public void testDesiredBalanceMetrics() {
         final var infoService = (InternalClusterInfoService) internalCluster().getCurrentMasterNodeInstance(ClusterInfoService.class);
         ClusterInfoServiceUtils.setUpdateFrequency(infoService, TimeValue.timeValueMillis(200));
         assertNotNull("info should not be null", ClusterInfoServiceUtils.refresh(infoService));
+        ClusterRerouteUtils.reroute(client()); // ensure we leverage the latest cluster info
 
         final var telemetryPlugin = getTelemetryPlugin(internalCluster().getMasterName());
         telemetryPlugin.collect();