code: public Mono<InstanceId> register(Registration registration) {
          Assert.notNull(registration, "'registration' must not be null");
          InstanceId id = generator.generateId(registration);
          Assert.notNull(id, "'id' must not be null");
          return repository.compute(id, (key, instance) -> {
              if (instance == null) {
                  instance = Instance.create(key);
              }
              return Mono.just(instance.register(registration));
          }).map(Instance::getId);
      }
docs: Register instance.
  
  @param registration instance to be registered.
  @return the if of the registered instance.