public String staticMemberId(String groupInstanceId) {
        if (groupInstanceId == null) return null;
        return staticMembers.get(groupInstanceId);
    }
Get member id of a static member that matches the given group
instance id.

@param groupInstanceId The group instance id.

@return The member id corresponding to the given instance id or null if it does not exist
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/consumer/ConsumerGroup.java