**Description**


It would be great if the metrics we publish in Airflow change to include TAGs for "variable" parts of those (such as dag_id, task_id, pool name) rather than being part of the metrics.

See here: https://airflow.apache.org/docs/1.10.12/metrics.html, https://airflow.readthedocs.io/en/latest/logging-monitoring/metrics.html  - those variable parts should be tags.

We might consider to change those metrics by default in 2.0 and possibly introduce a backport in 1.10.13 to allow people to migrate.


**Use case / motivation**

Having the variable parts of the metrics makes it really difficult to aggregate the metrics. Seems that statsd python library does not support tags, but there are possible solutions by installing extensions:

https://stackoverflow.com/questions/49852654/sending-statsd-metrics-with-tags-from-python-client

**Related Issues**

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Variable.set(key="arg1", value=[1, 2])
    with dag:
        task_2.expand(arg2=task_1()).operator
    
    
    serialized_dag = SerializedDAG.from_dict(SerializedDAG.to_dict(dag))
    dr.dag = serialized_dag
    
    ti = dr.get_task_instance(task_id="task_1")
    task1 = dag.get_task("task_1")
    ti.refresh_from_task(task1)
    ti.run()
    
    dr.task_instance_scheduling_decisions()
    tis = dr.get_task_instances()
    indices = [(ti.map_index, ti.state) for ti in tis if ti.map_index >= 0]
    assert sorted(indices) == [
        (0, State.NONE),
        (1, State.NONE),
        (2, TaskInstanceState.REMOVED),
    ]
def test_mapped_literal_faulty_state_in_db(dag_maker, session):
    """
    This test tries to recreate a faulty state in the database and checks if we can recover from it.
    The state that happens is that there exists mapped task instances and the unmapped task instance.
    So we have instances with map_index [-1, 0, 1]. The -1 task instances should be removed in this case.
    """
    with dag_maker(session=session) as dag:
        @task
        def task_1():