@VisibleForTesting
    static Map<String, String> populatePathVariables(ServiceRequestContext ctx,
                                                     List<PathVariable> pathVariables) {
        return pathVariables.stream().map(var -> {
            final String value =
                    var.values().stream()
                       .map(def -> {
                           if (def.type == Type.REFERENCE) {
                               return ctx.pathParam(def.value);
                           } else {
                               return def.value;
                           }
                       }).collect(Collectors.joining("/"));
            return new SimpleImmutableEntry<>(var.name(), value);
        }).collect(toImmutableMap(Entry::getKey, Entry::getValue));
    }
Converts the HTTP request to gRPC JSON with the {@link TranscodingSpec}.
grpc/src/main/java/com/linecorp/armeria/server/grpc/HttpJsonTranscodingService.java