Migrate native JSON encoding to serialize the protobuf data model instead of in-memory data model

[dsmith3197]

The motivation for this is to improve our backward/forward compatibility tests and guarantees. Once this is done, we can remove the existing test fixtures which are causing pain.

[tobz]

We'll need to also do a pass in our existing Protocol Buffers definitions and mark all of the older stuff as deprecated: the zillion versions of older metric types, and so on.

Building on that, we'll want to figure out some way that lets us avoid dragging those deprecated fields into anything we publish about what's accepted in the native JSON encoding.