public boolean hasStaticMember(String groupInstanceId) {
        return staticMembers.containsKey(groupInstanceId);
    }
Checks whether the given group instance id exists as
a static member.

@param groupInstanceId the group instance id.
@return true if a static member with the group instance id exists,
        false otherwise.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/classic/ClassicGroup.java