Add a label to indicate if DAG is paused or not

[nitvic793]

As the title suggests, it would be great if we can have some way to know if a DAG is paused or not, especially in the airflow_dag_run_duration metric, as the metric becomes useless if the a particular DAG is paused mid-run, then the DAG run duration just balloons up for that particular DAG.

The metric airflow_dag_status would also benefit from a paused="true"/"false" label, enabling easy filtering of DAGs in dashboards.