Use per-job aggregators about prometheus-gravel-gateway

[sinkingpoint]

Generally the prometheus model here is to namespace your metrics, so that no two jobs have the same metric name. e.g. you wouldn't have http_requests from two different services, but have service1_http_requests_total and service2_http_requests_total. That saves a log of headaches on the Prometheus side as well because you don't have to worry about alerts being poisoned by a new service treading on the metrics from an old one.

That being said, a per job delete job does make sense in #8