private Set<String> candidateProtocols() {
        // get the set of protocols that are commonly supported by all members
        return supportedProtocols.entrySet().stream()
            .filter(protocol -> protocol.getValue() == members.size())
            .map(Map.Entry::getKey).collect(Collectors.toSet());
    }
A candidate protocol must be supported by all members.

@return a set of candidate protocols that can be chosen as the protocol
        for the group.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/classic/ClassicGroup.java