'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.'

def add(self, metric_name, stat, config=None):

    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)
