Span new trace context for compute group task (elastic#125151)

We need to create a new trace context when registering a task. There are 
other issues with the task group, but I need this change to enable
allow_partial_results first, then follow up with other enhancements.

Author: dnhatn

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/plugin/ComputeService.java

@@ -435,11 +435,13 @@ Runnable cancelQueryOnFailure(CancellableTask task) {
 
     CancellableTask createGroupTask(Task parentTask, Supplier<String> description) throws TaskCancelledException {
         final TaskManager taskManager = transportService.getTaskManager();
-        return (CancellableTask) taskManager.register(
-            "transport",
-            "esql_compute_group",
-            new ComputeGroupTaskRequest(parentTask.taskInfo(transportService.getLocalNode().getId(), false).taskId(), description)
-        );
+        try (var ignored = transportService.getThreadPool().getThreadContext().newTraceContext()) {
+            return (CancellableTask) taskManager.register(
+                "transport",
+                "esql_compute_group",
+                new ComputeGroupTaskRequest(parentTask.taskInfo(transportService.getLocalNode().getId(), false).taskId(), description)
+            );
+        }
     }
 
     private static class ComputeGroupTaskRequest extends TransportRequest {