Improve on propolis metrics

[leftwo]

PR #147 introduces the Oximeter framework into propolis-server.
The current (single) metric in propolis that is registered with Oximeter can be improved upon.

[gjcolombo]

Triaging to Unscheduled. We can certainly record more stuff in general, but I don't think there are any specific metrics gaps we're currently planning to fill (or know that we must fill before we can ship).