Registry stats

This MR does two things. The first is that it adds registry posting and getting to the performance stats that get tracked in the API. And then the second is that it extends the performance tracking to also track raw throughput coming through the api, so that the fleet manager can get a sense of how much traffic over the past few minutes each server and the whole fleet has been doing. This will make it easier for the fleet manager to highlight places where servers seem to be struggling.

One thing that is missing from this MR and will need to be done in a future MR is the ability to measure how much backlog a server has. For example, it'd be great to see that the server has 20 download requests pending that it's trying to get through. Or that it's got 5 pending registry writes, etc.