Expose deprecated option in interceptor

[makasim]

Is your feature request related to a problem? Please describe.
It would allow us to implement an interceptor that logs the usage of deprecated endpoints.

Describe the solution you'd like

RPC specs provide a deprecated option that could be used on a method. Currently, Connect only added a deprecated comment. Exposing the value to interceptors could help to build more robust solutions.

service CreateService {
  rpc Create(CreateRequest) returns (CreateResponse) {
    option deprecated = true;
  }
}

The solution I see: The option could be added to connect.Spec. That would be enough for this particular case.

WrapUnary(unaryFunc connect.UnaryFunc) connect.UnaryFunc {
	return func(ctx context.Context, request connect.AnyRequest) (connect.AnyResponse, error) {
	    if request.Spec().Deprecated {
	       log.Printf("WARNING: %s is deprecated", request.Spec().Procedure)
	    }
            return unaryFunc(ctx, request)
	}
}

A more generic approach is to expose all the options in connect.Spec somehow (map?). In this case, we could add a custom option with a api removal date and set some alerts for it.

Describe alternatives you've considered

Additional context

[akshayjshah]

Hey @makasim! Great suggestion - I've been meaning to expose the RPC's descriptor on Spec, which would give you access to deprecated, any custom options, and a bunch of other info.

In the meantime, you can get the descriptor yourself from the global registry: see this comment on another issue. protoc-gen-connect-go now generates constants for service names, so you shouldn't need a hard-coded string to look up the descriptor.

[makasim]

Looks like a global registry would solve my issue. Thank you for the hint!