// returns a string telling the error, or "" if no error
public string AddMetric(Activity activity, Metric metric)
{
if (activity.MetricForName(metric.Name) != null)
{
return("Activity " + activity.Name + " already has metric " + metric.Name);
}
activity.AddIntrinsicMetric(metric);
this.MetricAdded.Invoke(metric, activity);
return("");
}