feat: proxied models (remotely learned models used in DefaultRegistry)

[milanlenco]

This small PR allows to register models learned remotely (i.e. over gRCP using the meta service) into the LocalRegistry.

Why is this useful?
This change can be used as a building block for integration of multiple (distributed) CNFs (pairs of agent + some data-plane component) into one appliance with a single API endpoint. A central agent can dynamically discover all CNFs, learn their models over gRPC and generate kv descriptors for them that would mostly just forward CRUD operations via remoteclient.
So basically the change allows the following configuration sequence:

agentctl/etcd/... -> [proxy agent(s) ->]  target agent

To proxy a given model one would write something like:

client, err = remoteclient.NewClientGRPC(grpcConn, remoteclient.UseRemoteRegistry("config"))
models, err := swMod.cfgClient.KnownModels("config")
for _, model := range models {
    if !proxyThisModel {
      continue
    }
    spec := models.ToSpec(cnfModel.info.Spec)
    _, err := models.DefaultRegistry.Register(cnfModel.info, spec)
    // build descriptor for it, etc...
}

Signed-off-by: Milan Lenco milan.lenco@pantheon.tech

[fgschwan]

This call is the same as one above. I understand that the first one is for existence checking and this one is for model itself, but lets call it only once and just make a note in comment that "err != nil" check also for nonexistence of model.

[milanlenco]

done

[fgschwan]

The syntax of the "x" paramater doesn't inforce it, but the documentation says that the protobuf message is expected. So i would prefer a proto message as input. In your case you can use this to create new empty proto message:
dynamicpb.NewMessageType(modelinfo.MessageDescriptor).New().Interface()
(maybe you can put it somewhere as conversion helper function)
This will create a dynamicpb.Message instance and you can use that type here as type check switch.

[milanlenco]

I can update the function documentation, but I don't think that I can use dynamicpb here since RemoteRegistry requires ModelInfo.

[fgschwan]

Unfortunately true. So please just update to docs (the registry interface docs probably also need some update).

[milanlenco]

done