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.

def transmit_metrics(self):
        """
        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.
        """
        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()