Support for spring Converter beans for @PathVariables

I would like to be able to use custom value objects in my feign clients.

For example:

@FeignClient
public interface MyClient {

  @GetMapping("/users/{userId}")
  User getUser(@PathVariable UserId userId);
}

Note how the @PathVariable is typed to UserId, not to String.

I tried adding a org.springframework.core.convert.converter.Converter<UserId,String> in the application context that converts from UserId to String, but that was not used. The toString() method of UserId was still used instead.

Start by tracing how @PathVariable values are converted in the Spring OpenFeign integration and how application-context Converter<UserId,String> beans are discovered. Reproduce the example with a custom UserId and verify that the registered Converter is used instead of toString(); add coverage showing the expected path value.