code: def transmit_metrics(self):
          
          global _last_stats_transmit_time  # pylint: disable=global-statement
          with _STATS_LOCK:  # pylint: disable=not-context-manager
              if time.monotonic() - _last_stats_transmit_time < 10.0:
                  return
  
              for site in self.state:
                  for filetype, prop in self.state[site]["upload"].items():
                      if prop["last_success"]:
                          self.metrics.gauge(
                              "pghoard.last_upload_age",
                              time.monotonic() - prop["last_success"],
                              tags={
                                  "site": site,
                                  "type": filetype,
                              }
                          )
              _last_stats_transmit_time = time.monotonic()
docs: Keep metrics updated about how long time ago each filetype was successfully uploaded.
          Transmits max once per ten seconds, regardless of how many threads are running.