docs: @GetMapping(path = "/instances/{id}", produces = MediaType.APPLICATION_JSON_VALUE)
      public Mono<ResponseEntity<Instance>> instance(@PathVariable String id) {
          LOGGER.debug("Deliver registered instance with ID '{}'", id);
          return registry.getInstance(InstanceId.of(id))
                         .filter(Instance::isRegistered)
                         .map(ResponseEntity::ok)
                         .defaultIfEmpty(ResponseEntity.notFound().build());
      }