code: @Override
      public void addProperty(String propertyName, Object value) {
          List<Object> propertyList = properties.get(propertyName);
          if (propertyList == null) {
              propertyList = new ArrayList<Object>();
              properties.put(propertyName, propertyList);
          }
          
          propertyList.add(value);
      }
docs: Add a property. If you add the same property more than once, it stores
  all values a list.
  
  This feature is enabled if the system property
  "com.amazonaws.sdk.enableRuntimeProfiling" is set, or if a
  {@link RequestMetricCollector} is in use either at the request, web service
  client, or AWS SDK level.
  
  @param propertyName
  The name of the property
  @param value
  The property value