As discussed in various previous issues, e.g. #3155, and my post at https://blog.envoyproxy.io/dynamic-extensibility-and-protocol-buffers-dcd0bf0b8801, we want to head towards using Any as the canonical extension type. This is more efficient on the wire and we're now seeing use cases (e.g. Istio CC @costinm) where Any would be bring a real pay-off.

We will continue to support Struct embedded inside an Any for those folks who prefer working with such types.

Rather than adhoc switching to Any, we should aim to maintain a consistent API theme around opaque extension configuration.

My recommendation is this:

1. We come up with a clean way to switch to Any and turndown the use of Struct for extensions. This might be just adding in an Any replacement with a oneof with the existing Struct uses.
2. We do this across all APIs in one sweep. We tag the Struct opaque data as deprecated. All future extension APIs only use Any.
3. We support Structs embedded in Any fields in the implementation, to provide flexibility to folks who want to continue using Struct.

With the right use of templated conversion utils, I think this can be done without a lot of boiler plate.