Conf settings to disable unwanted metric types, and add set types

This pull request has two changes (sorry):

#### Config settings to disable unwanted metric types
* I didn't need both metric counts and rates, so I wanted a config flag for it. While I was there, I added flags for the rest of the metrics.
* Everything defaults to true because it seemed like the least surprising thing to do. I'm not sold on sending counters *and* rates for a metric, but I don't see a better default than "everything on."
* I haven't added examples of this to Readme.md -- let me know if you'd like me to add that before pulling.

#### Set metrics
* We've been using them for a month with this patch to track concurrent users, seems solid.