docs: public <G extends Serializable> Metric<G> getMetric(CxxMetricsFactory.Key metricKey) {
      Metric<G> metric = (Metric<G>) this.langSpecificMetrics.get(metricKey);
      if (metric == null) {
        throw new IllegalStateException("Requested metric " + metricKey + " couldn't be found");
      }
      return metric;
    }