docs: def avg_time(self, source=None):
          """
          Returns the average time taken to scrape lyrics. If a string or a
          function is passed as source, return the average time taken to scrape
          lyrics from that source, otherwise return the total average.
          """
          if source is None:
              runtimes = []
              for rec in self.source_stats.values():
                  runtimes.extend([r for r in rec.runtimes if r != 0])
              return avg(runtimes)
          else:
              if callable(source):
                  return avg(self.source_stats[source.__name__].runtimes)
              else:
                  return avg(self.source_stats[source].runtimes)