def increment(self, counter_name, delta):
    """Increment counter value.

    Args:
      counter_name: counter name as String.
      delta: increment delta as Integer.

    Returns:
      new counter value.
    """
    current_value = self.counters.get(counter_name, 0)
    new_value = current_value + delta
    self.counters[counter_name] = new_value
    return new_value