add metrics on jobs count per worker type
See #2095.

Add metrics `worker_size_jobs_count{pid="10",worker_size="heavy"}: 12` to prometheus (value for worker_size: `heavy`, `medium` and `light`).

It will be used to autoscale the workers 

Also:
- difficulty_min is now excluded (and 0 is not a valid difficulty anymore)
- in prod, medium and heavy workers will now only process jobs for their range, not smaller ones

add metrics on jobs count per worker type: See #2095.

Add metrics `worker_size_jobs_count{pid="10",worker_size="heavy"}: 12` to prometheus (value for worker_size: `heavy`, `medium` and `light`).

It will be used to autoscale the workers 

Also:
- difficulty_min is now excluded (and 0 is not a valid difficulty anymore)
- in prod, medium and heavy workers will now only process jobs for their range, not smaller ones
