Could you add a new feature to support query the celery task length

[wanghsinche]

Description

• What is the motivation / use case for changing the behavior? it is important to monitor the queue length for preventing memory overflow. could you kindly add some method to get the current task queue length? it'd be usefully in production environment.

• Proposed Behavior

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, gitter, etc)