Small cleanup in HealthMetadataService (elastic#98197)

Tidies up the nested task/executor classes to remove some indirection,
and implements `describeTasks()` to avoid noise in debug logs.

server/src/main/java/org/elasticsearch/health/metadata/HealthMetadataService.java

@@ -67,11 +67,7 @@ private HealthMetadataService(ClusterService clusterService, Settings settings)
         this.clusterStateListener = this::updateOnClusterStateChange;
         this.enabled = ENABLED_SETTING.get(settings);
         this.localHealthMetadata = initialHealthMetadata(settings);
-        this.taskQueue = clusterService.createTaskQueue(
-            "health metadata service",
-            Priority.NORMAL,
-            new UpsertHealthMetadataTask.Executor()
-        );
+        this.taskQueue = clusterService.createTaskQueue("health metadata service", Priority.NORMAL, new Executor());
     }
 
     public static HealthMetadataService create(ClusterService clusterService, Settings settings) {
@@ -128,7 +124,7 @@ private void updateOnHealthNodeEnabledChange(boolean enabled) {
             clusterService.addListener(clusterStateListener);
 
             if (canPostClusterStateUpdates(clusterService.state())) {
-                taskQueue.submitTask("health-node-enabled", () -> this.localHealthMetadata, null);
+                taskQueue.submitTask("health-node-enabled", new UpsertHealthMetadataTask(), null);
             }
         } else {
             clusterService.removeListener(clusterStateListener);
@@ -151,8 +147,8 @@ private void updateOnClusterStateChange(ClusterChangedEvent event) {
             this.isMaster = event.localNodeMaster();
         }
         if (canPostClusterStateUpdates(event.state())) {
-            if (this.localHealthMetadata.equals(HealthMetadata.getFromClusterState(event.state())) == false) {
-                taskQueue.submitTask("store-local-health-metadata", () -> this.localHealthMetadata, null);
+            if (localHealthMetadata.equals(HealthMetadata.getFromClusterState(event.state())) == false) {
+                taskQueue.submitTask("store-local-health-metadata", new UpsertHealthMetadataTask(), null);
             }
         }
     }
@@ -167,36 +163,36 @@ public static List<NamedWriteableRegistry.Entry> getNamedWriteables() {
     /**
      * A base class for health metadata cluster state update tasks.
      */
-    interface UpsertHealthMetadataTask extends ClusterStateTaskListener {
-
+    private static class UpsertHealthMetadataTask implements ClusterStateTaskListener {
         @Override
-        default void onFailure(@Nullable Exception e) {
+        public void onFailure(@Nullable Exception e) {
             logger.log(
                 MasterService.isPublishFailureException(e) ? Level.DEBUG : Level.WARN,
                 () -> "failure during health metadata update",
                 e
             );
         }
+    }
 
-        default ClusterState execute(ClusterState currentState) {
-            var initialHealthMetadata = HealthMetadata.getFromClusterState(currentState);
-            var finalHealthMetadata = latestLocalMetadata();
-            return finalHealthMetadata.equals(initialHealthMetadata)
-                ? currentState
-                : currentState.copyAndUpdate(b -> b.putCustom(HealthMetadata.TYPE, finalHealthMetadata));
+    private class Executor extends SimpleBatchedExecutor<UpsertHealthMetadataTask, Void> {
+        @Override
+        public Tuple<ClusterState, Void> executeTask(UpsertHealthMetadataTask task, ClusterState clusterState) {
+            final var initialHealthMetadata = HealthMetadata.getFromClusterState(clusterState);
+            final var finalHealthMetadata = localHealthMetadata; // single volatile read
+            return Tuple.tuple(
+                finalHealthMetadata.equals(initialHealthMetadata)
+                    ? clusterState
+                    : clusterState.copyAndUpdate(b -> b.putCustom(HealthMetadata.TYPE, finalHealthMetadata)),
+                null
+            );
         }
 
-        HealthMetadata latestLocalMetadata();
-
-        class Executor extends SimpleBatchedExecutor<UpsertHealthMetadataTask, Void> {
-
-            @Override
-            public Tuple<ClusterState, Void> executeTask(UpsertHealthMetadataTask task, ClusterState clusterState) {
-                return Tuple.tuple(task.execute(clusterState), null);
-            }
+        @Override
+        public void taskSucceeded(UpsertHealthMetadataTask task, Void unused) {}
 
-            @Override
-            public void taskSucceeded(UpsertHealthMetadataTask task, Void unused) {}
+        @Override
+        public String describeTasks(List<UpsertHealthMetadataTask> tasks) {
+            return ""; // tasks are equivalent and idempotent, no need to list them out
         }
     }