@TestOnly
  protected synchronized Collection<Subject> getAllSubjects() {
    List<Subject> all = uniqueIdVsSubjects.values().stream()
        .flatMap(List::stream)
        .collect(Collectors.toList());
    return Collections.unmodifiableCollection(all);
  }
This is per ServerConnection or per CacheClientProxy, corresponding to only one client
connection.
Credentials should usually be just one, only multiple in multi-user case.
geode-core/src/main/java/org/apache/geode/internal/cache/tier/sockets/ClientUserAuths.java