docs: def add(self, metric_name, stat, config=None):
          """
          Register a metric with this sensor
  
          Arguments:
              metric_name (MetricName): The name of the metric
              stat (AbstractMeasurableStat): The statistic to keep
              config (MetricConfig): A special configuration for this metric.
                  If None use the sensor default configuration.
          """
          with self._lock:
              metric = KafkaMetric(metric_name, stat, config or self._config)
              self._registry.register_metric(metric)
              self._metrics.append(metric)
              self._stats.append(stat)