Fix test in TransportGetAllocationStatsActionTests

testReturnsOnlyRequestedStats() was using
EnumSet.copyOf(randomSubsetOf(Metric.values().length, Metric.values()))
which will always return all of the metrics.  The code was expecting
Metric.ALLOCATIONS and Metric.FS to sometimes not be in the returned set.
This change uses an explicit list of EnumSets to cover the range of
scenarios expected in the test code.

Author: JeremyDahlgren

server/src/test/java/org/elasticsearch/action/admin/cluster/allocation/TransportGetAllocationStatsActionTests.java

@@ -29,6 +29,7 @@
 import org.junit.Before;
 
 import java.util.EnumSet;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.CyclicBarrier;
@@ -41,6 +42,7 @@
 import static org.hamcrest.Matchers.not;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
@@ -87,33 +89,42 @@ public void tearDown() throws Exception {
     }
 
     public void testReturnsOnlyRequestedStats() throws Exception {
+        int expectedNumberOfStatsServiceCalls = 0;
+
+        for (final var metrics : List.of(
+            EnumSet.of(Metric.ALLOCATIONS, Metric.FS),
+            EnumSet.of(Metric.ALLOCATIONS),
+            EnumSet.of(Metric.FS),
+            EnumSet.noneOf(Metric.class),
+            EnumSet.allOf(Metric.class)
+        )) {
+            var request = new TransportGetAllocationStatsAction.Request(
+                TimeValue.ONE_MINUTE,
+                new TaskId(randomIdentifier(), randomNonNegativeLong()),
+                metrics
+            );
+
+            when(allocationStatsService.stats()).thenReturn(
+                Map.of(randomIdentifier(), NodeAllocationStatsTests.randomNodeAllocationStats())
+            );
+
+            var future = new PlainActionFuture<TransportGetAllocationStatsAction.Response>();
+            action.masterOperation(mock(Task.class), request, ClusterState.EMPTY_STATE, future);
+            var response = future.get();
+
+            if (metrics.contains(Metric.ALLOCATIONS)) {
+                assertThat(response.getNodeAllocationStats(), not(anEmptyMap()));
+                verify(allocationStatsService, times(++expectedNumberOfStatsServiceCalls)).stats();
+            } else {
+                assertThat(response.getNodeAllocationStats(), anEmptyMap());
+                verify(allocationStatsService, times(expectedNumberOfStatsServiceCalls)).stats();
+            }
 
-        var metrics = EnumSet.copyOf(randomSubsetOf(Metric.values().length, Metric.values()));
-
-        var request = new TransportGetAllocationStatsAction.Request(
-            TimeValue.ONE_MINUTE,
-            new TaskId(randomIdentifier(), randomNonNegativeLong()),
-            metrics
-        );
-
-        when(allocationStatsService.stats()).thenReturn(Map.of(randomIdentifier(), NodeAllocationStatsTests.randomNodeAllocationStats()));
-
-        var future = new PlainActionFuture<TransportGetAllocationStatsAction.Response>();
-        action.masterOperation(mock(Task.class), request, ClusterState.EMPTY_STATE, future);
-        var response = future.get();
-
-        if (metrics.contains(Metric.ALLOCATIONS)) {
-            assertThat(response.getNodeAllocationStats(), not(anEmptyMap()));
-            verify(allocationStatsService).stats();
-        } else {
-            assertThat(response.getNodeAllocationStats(), anEmptyMap());
-            verify(allocationStatsService, never()).stats();
-        }
-
-        if (metrics.contains(Metric.FS)) {
-            assertNotNull(response.getDiskThresholdSettings());
-        } else {
-            assertNull(response.getDiskThresholdSettings());
+            if (metrics.contains(Metric.FS)) {
+                assertNotNull(response.getDiskThresholdSettings());
+            } else {
+                assertNull(response.getDiskThresholdSettings());
+            }
         }
     }