Given two 'members' elements from rs.status(),
            return lag between their optimes (in secs).

def get_repl_lag(self, master_status):
        """
            Given two 'members' elements from rs.status(),
            return lag between their optimes (in secs).
        """
        member_status = self.get_member_rs_status()

        if not member_status:
            raise MongoctlException("Unable to determine replicaset status for"
                                    " member '%s'" %
                                    self.id)

        return get_member_repl_lag(member_status, master_status)