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