code: def __add__(self, other: Metric):
      if other is None:
          return self
      counter = self._counter + other._counter
      return type(self)(counter)
  
imports: from __future__ import annotations
  from typing import Tuple
  from collections import Counter
  from parlai.core.torch_agent import TorchAgent
  from parlai.core.metrics import Metric, SumMetric
  from parlai.core.message import Message
label_desc: Use image node_id 2 for calling the _CounterMetric obj's underlying member method code with example usage: obj.__add__(other) and returns: type, self