Mbq.metrics

- Added the `namespace` kwarg and removed the `statsd` kwarg from the `Collector` constructor. The `statsd` instance used by the Collector can no longer be overridden, but we _add_ the option to override the namespace used by the `statsd` instance.

This update also makes the following minor change:
- Tags are now de-duped in the code. Tags you pass in to `increment, event..` get merged with the collector tags which get merged with the global default tags set by the `init` function. This could almost be considered a bug fix, as I doubt anybody expected that we could send two tags with the same name.

See usage in Isengard

- Simplified metric names and datadog tags.
Ex: `response-time.http-request-path` -> `response.path`
- Replaced underscores in metric names with dashes. Datadog converts dashes to underscores anyways.
- Added http reponse status range tag (4xx, 5xx etc).
- Before we were only using `metrics.timing` with namespace `response-time`. I separated out all the non-timing tags into a `metrics.increment` call with namespace `response`. The timing call will use namespace `request_duration_ms`. This change mirrors the names/tags in os-core used in the [oscore datadog dashboard](https://app.datadoghq.com/dash/811835/os-core?live=true&page=0&is_auto=false&from_ts=1534788937054&to_ts=1534792537054&tile_size=m).