Fix the issue of missing implementation of IndexerTaskCountStatsProvider for peons (#16875)

Bug description:
Peons to fail to start up when `WorkerTaskCountStatsMonitor` is used on MiddleManagers.
This is because MiddleManagers pass on their properties to peons and peons are unable to
find `IndexerTaskCountStatsProvider` as that is bound only for indexer nodes.

Fix:
Check if node is an indexer before trying to get instance of `IndexerTaskCountStatsProvider`.

server/src/main/java/org/apache/druid/server/metrics/WorkerTaskCountStatsMonitor.java

@@ -38,6 +38,7 @@ public class WorkerTaskCountStatsMonitor extends AbstractMonitor
   private final String workerCategory;
   private final String workerVersion;
   private final boolean isMiddleManager;
+  private final boolean isIndexer;
 
   @Inject
   public WorkerTaskCountStatsMonitor(
@@ -46,16 +47,22 @@ public class WorkerTaskCountStatsMonitor extends AbstractMonitor
   )
   {
     this.isMiddleManager = nodeRoles.contains(NodeRole.MIDDLE_MANAGER);
+    this.isIndexer = nodeRoles.contains(NodeRole.INDEXER);
     if (isMiddleManager) {
       this.statsProvider = injector.getInstance(WorkerTaskCountStatsProvider.class);
       this.indexerStatsProvider = null;
       this.workerCategory = statsProvider.getWorkerCategory();
       this.workerVersion = statsProvider.getWorkerVersion();
-    } else {
+    } else if (isIndexer) {
       this.indexerStatsProvider = injector.getInstance(IndexerTaskCountStatsProvider.class);
       this.statsProvider = null;
       this.workerCategory = null;
       this.workerVersion = null;
+    } else {
+      this.indexerStatsProvider = null;
+      this.statsProvider = null;
+      this.workerCategory = null;
+      this.workerVersion = null;
     }
   }
 
@@ -68,7 +75,7 @@ public class WorkerTaskCountStatsMonitor extends AbstractMonitor
       emit(emitter, "worker/taskSlot/idle/count", statsProvider.getWorkerIdleTaskSlotCount());
       emit(emitter, "worker/taskSlot/total/count", statsProvider.getWorkerTotalTaskSlotCount());
       emit(emitter, "worker/taskSlot/used/count", statsProvider.getWorkerUsedTaskSlotCount());
-    } else {
+    } else if (isIndexer) {
       emit(emitter, "worker/task/running/count", indexerStatsProvider.getWorkerRunningTasks());
       emit(emitter, "worker/task/assigned/count", indexerStatsProvider.getWorkerAssignedTasks());
       emit(emitter, "worker/task/completed/count", indexerStatsProvider.getWorkerCompletedTasks());

server/src/test/java/org/apache/druid/server/metrics/WorkerTaskCountStatsMonitorTest.java

@@ -309,6 +309,7 @@ public class WorkerTaskCountStatsMonitorTest
             89L
     );
   }
+
   @Test
   public void testMonitorWithNulls()
   {
@@ -318,4 +319,14 @@ public class WorkerTaskCountStatsMonitorTest
     monitor.doMonitor(emitter);
     Assert.assertEquals(0, emitter.getEvents().size());
   }
+
+  @Test
+  public void testMonitorWithPeon()
+  {
+    final WorkerTaskCountStatsMonitor monitor =
+            new WorkerTaskCountStatsMonitor(injectorForPeon, ImmutableSet.of(NodeRole.PEON));
+    final StubServiceEmitter emitter = new StubServiceEmitter("service", "host");
+    monitor.doMonitor(emitter);
+    Assert.assertEquals(0, emitter.getEvents().size());
+  }
 }