public boolean add(MetricName metricName, MeasurableStat stat) {
        return add(metricName, stat, null);
    }
Register a metric with this sensor
@param metricName The name of the metric
@param stat The statistic to keep
@return true if metric is added to sensor, false if sensor is expired
clients/src/main/java/org/apache/kafka/common/metrics/Sensor.java