code: public final void updateMetric(MetricName name, Object value) {
          if (!started) {
              return;
          }
          PcpValueInfo info = metricData.get(name);
          if (info == null) {
              throw new IllegalArgumentException("Metric " + name
                      + " was not added before initialising the writer");
          }
          updateValue(info, value);
      }
docs: /*
  (non-Javadoc)
  @see io.pcp.parfait.pcp.PcpWriter#updateMetric(java.lang.String, java.lang.Object)