v2.0.0

New metrics for successful and failed job counts and worker total working time in seconds, used to calculate the avg. job latency, added by @mkotsalainen.

Metric Name	Type	Labels	Description
rq_workers_success_total	Counter	name, queues	Successful job count by worker
rq_workers_failed_total	Counter	name, queues	Failed job count by worker
rq_workers_working_time_total	Counter	name, queues	Total working time in seconds by worker