code: def add_link(self, link):
          
          if isinstance(link, link_module.Link):
              self.links.append(link)
          else:
              raise TypeError("Type Error: received {}, but requires Link.".
                              format(type(link).__name__))
docs: Add a Link.
  
          :type link: :class: `~opencensus.trace.link.Link`
          :param link: A Link object.