docs: def add_metric(self, metric_name, measurable, config=None):
          """
          Add a metric to monitor an object that implements measurable.
          This metric won't be associated with any sensor.
          This is a way to expose existing values as metrics.
  
          Arguments:
              metricName (MetricName): The name of the metric
              measurable (AbstractMeasurable): The measurable that will be
                  measured by this metric
              config (MetricConfig, optional): The configuration to use when
                  measuring this measurable
          """
          # NOTE there was a lock here, but i don't think it's needed
          metric = KafkaMetric(metric_name, measurable, config or self.config)
          self.register_metric(metric)