'Increment counter value.
Args:
counter_name: counter name as String.
delta: increment delta as Integer.
Returns:
new counter value.'

def increment(self, counter_name, delta):

    current_value = self.counters.get(counter_name, 0)
    new_value = (current_value + delta)
    self.counters[counter_name] = new_value
    return new_value
