public Set<DomainName> getHostnames(ApplicationId applicationId) {
        HashSet<DomainName> set = hostnamesById.get(applicationId);
        return set == null ? Set.of() : Set.copyOf(set);
    }
Note: Returns an empty set for unknown application.
service-monitor/src/main/java/com/yahoo/vespa/service/duper/DuperModel.java