Add datasource as label to prometheus metric

[thomas-dufour]

Today, the available labels are cluster, container, endpoint, instance, job, namespace, pod, prometheus, runner_status, service and type. It would be great to add the datasource to the labels as it would enable easy monitoring of Druid's ingestion progression for a given customer.

It seems easily feasible:

• we can change the query https://github.com/wiremind/druid-tasks-exporter/blob/main/main.go#L44C12-L44C99 by
  SELECT type,runner_status,datasource,count(*) AS total FROM sys.tasks GROUP BY type,runner_status,datasource