code: @Reference(policy = ReferencePolicy.DYNAMIC, cardinality = ReferenceCardinality.MULTIPLE, target = "(config.displayId=*)")
      protected Map<String, Object> setUserRegistry(ServiceReference<UserRegistryService> ref) {
          adjustUserRegistryServiceRef(ref);
  
          // determine a new user registry service
          userRegistryService.set(null);
          return getServiceProperties();
      }
docs: Method will be called for each UserRegistryService that is registered
  in the OSGi service registry. We maintain an internal map of these for
  easy access.
  
  @param ref Reference to a registered UserRegistryService