Choosing the right metric_type mapping and aggregator function for the Airflow dashboard's `scheduler.heartbeat` field in the context of TSDB.

Previously, the sum aggregator function was used to display `scheduler.heartbeat` in the Airflow dashboard before TSDB integration. Although the sum aggregator isn't ideal for representing this field, it was switched to max to address visualization issues. This was done as sum, avg aggregators, etc., do not align well with counter fields.
It's required to update an appropriate aggregator to accurately represent this field in TSDB-enabled mode too.