Changes the way metrics from the middleware are sent to Datadog. Before they had namespaces like `<servicename>.request`, so that each service needed a separate monitor. The metrics namespaces are now`mbq.metrics.connections`, `mbq.metrics.request_duration_ms`, and `mbq.metrics.connections`. This way all services using mbq.metrics can share the same Datadog monitors for request metrics. This is consistent with how we do monitors for mbq.pubsub, mbq.ranch, and mbq.atomiq.
This change also makes a breaking change to the `init` function, which now requires a `service` argument, as well as an `Environment` object from `mbq.env`.