Design the Mixer's Direct API

[geeknoid]

The Mixer currently exposes an API intended for use by the proxy (the intermediated API). We wish to allow services to directly call Mixer in order to do:

• Custom reporting (metrics, logs, tracing)
• Custom authz checks (permissions on resources)
• Custom quota operations.

We want to design this in the 0.3 timeframe with an eye towards implementation in 0.4. In addition to Mixer's own API, we need to understand the dependency on client libraries as part of the end-to-end direct API story.

[douglas-reid]

High-level design doc proposal: https://goo.gl/3KoVf7