Separate web from worker metrics and add 2 more

[hammady]

• This separates Prometheus metrics into web and worker metrics
• For web, it adds response time metric
• For worker, it adds queue size metric
• It also fixes an issue where the Prometheus instance label was not unique, and typically it should match the instance unique identifier (e.g. hostname).

[coveralls]

Coverage decreased (-1.5%) to 73.919% when pulling 64f1dcf on queue-size-reporting into 3d13dd9 on master.