Use rack.exception as metric name.

I'm going to move the `exception` metric into an [exceptions](https://github.com/remind101/exceptions) handler, so our metrics would be:

* `exception`: Incremented on any call to `Exceptions.notify`
* `rack.exception`: Incremented when there's an exception within a rack request.
* `sidekiq.exception`: Incremented when there's an exception within a sidekiq job.