Add celery-exporter

[danihodovic]

Metrics

Name	Description	Type
celery_task_sent_total	Sent when a task message is published.	Counter
celery_task_received_total	Sent when the worker receives a task.	Counter
celery_task_started_total	Sent just before the worker executes the task.	Counter
celery_task_succeeded_total	Sent if the task executed successfully.	Counter
celery_task_failed_total	Sent if the execution of the task failed.	Counter
celery_task_rejected_total	The task was rejected by the worker, possibly to be re-queued or moved to a dead letter queue.	Counter
celery_task_revoked_total	Sent if the task has been revoked.	Counter
celery_task_retried_total	Sent if the task failed, but will be retried in the future.	Counter
celery_worker_up	Indicates if a worker has recently sent a heartbeat.	Gauge
celery_worker_tasks_active	The number of tasks the worker is currently processing	Gauge

[danihodovic]

@RichiH could you have a look at this one as well? It's more widely used than the borg-exporter I wrote.

[RichiH]

Minor nits/comments:

• It would be more customary to use tasks.
• Can celery_task_received_total be calculated from the other metrics?
• Is celery_worker_up about one single worker or a group of workers?

Can you attach sample output from the exporter, please?