public Counter Unregister(IInstrumentTarget target)
 {
     if (target == null)
     {
         throw new ArgumentNullException("target");
     }
     instrumentTargets.Remove(target);
     return(this);
 }
 public Counter Register(IInstrumentTarget target)
 {
     if (target == null)
     {
         throw new ArgumentNullException();
     }
     instrumentTargets.Add(target);
     return(this);
 }