representation of the current client), the current `PushContext` snapshot, and a list of config updates that caused the change.

The `Proxy` as an input parameter is important, and a major distinction from some other XDS implementations. We are not able to statically translate inputs to XDS without per-client information. For example, we rely on the client's labels to determine the set of policies applied. While this is necessary to implement Istio's APIs, it does limit performance substantially....