Fix testSkipRefreshIfShardIsRefreshingAlready (#50856)

The test checked queue size and active count, however,
ThreadPoolExecutor pulls out the request from the queue before marking
the worker active, risking that we think all tasks are done when they
are not. Now check on completed-tasks metric instead, which is
guaranteed to be monotonic.

Relates #50769

server/src/test/java/org/elasticsearch/indices/IndexingMemoryControllerTests.java

@@ -432,19 +432,18 @@ public class IndexingMemoryControllerTests extends IndexShardTestCase {
             }
         };
         int iterations = randomIntBetween(10, 100);
+        ThreadPoolStats.Stats beforeStats = getRefreshThreadPoolStats();
         for (int i = 0; i < iterations; i++) {
             controller.forceCheck();
         }
         assertBusy(() -> {
             ThreadPoolStats.Stats stats = getRefreshThreadPoolStats();
-            assertThat(stats.getQueue(), equalTo(0));
-            assertThat(stats.getActive(), equalTo(1));
+            assertThat(stats.getCompleted(), equalTo(beforeStats.getCompleted() + iterations - 1));
         });
         refreshLatch.get().countDown(); // allow refresh
         assertBusy(() -> {
             ThreadPoolStats.Stats stats = getRefreshThreadPoolStats();
-            assertThat(stats.getQueue(), equalTo(0));
-            assertThat(stats.getActive(), equalTo(0));
+            assertThat(stats.getCompleted(), equalTo(beforeStats.getCompleted() + iterations));
         });
         assertThat(shard.refreshStats().getTotal(), equalTo(refreshStats.getTotal() + 1));
         closeShards(shard);