**Description**
In its current shape the ExternalTaskSensor accepts either a single task_id or None to poll for the completion of a dag run. We have a use case where a dependent dag should poll for only certain list of tasks in the upstream dag. One option is to add N ExternalTaskSensor nodes if there are N nodes to be dependent on but those will be too many Sensor Nodes in the dag and can be avoided if the ExternalTaskSensor can accept a list of task_ids to poll for. 


**Use case / motivation**

We have a upstream dag that updates a list of hive tables that is further used by a lot of downstream dags. This dag updates 100s of hive tables but some of the downstream dag depends only upon 10-20 of these tables, There are multiple dags which depends upon varied list of hive tables from the upstream dag. 


**Are you willing to submit a PR?**

Yes, we are willing to submit a PR for this

**Related Issues**

Not that I am aware of . I did a search on the issue list. 

tests/sensors/test_external_task_sensor.py
"""
    Clearing across multiple DAGs should raise AirflowException if more levels are being cleared
    than allowed by the recursion_depth of the first ExternalTaskMarker being cleared.
    """
    run_tasks(dag_bag_ext)
    dag_0 = dag_bag_ext.get_dag("dag_0")
    task_a_0 = dag_0.get_task("task_a_0")
    task_b_0 = dag_0.get_task("task_b_0")
    task_b_0.recursion_depth = 2
    with pytest.raises(AirflowException, match="Maximum recursion depth 2"):
        clear_tasks(dag_bag_ext, dag_0, task_a_0)
@pytest.fixture
def dag_bag_cyclic():