public byte[] metadata(String protocolName) {
        for (JoinGroupRequestProtocol protocol : supportedProtocols) {
            if (protocol.name().equals(protocolName)) {
                return protocol.metadata();
            }
        }

        throw new IllegalArgumentException("Member does not support protocol " +
            protocolName);
    }
Get the metadata corresponding to the provided protocol.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/classic/ClassicGroupMember.java