code: def metric_key(self):
      return "sys_utt_self_bleu3"
  
imports: from parlai.core.message import Message
  from parlai.core.metrics import Metric, AverageMetric, normalize_answer, BleuMetric, SumMetric
  from parlai.core.tod.tod_core import STANDARD_API_NAME_SLOT, STANDARD_REQUIRED_KEY, STANDARD_OPTIONAL_KEY, STANDARD_API_SCHEMAS
  from typing import Dict, List, Optional, Tuple
  from parlai.core.tod.world_metrics_handlers import TodMetricsHandler, register_metrics_handler, _ApiCallGoalInteractionHelper, goals_hit_helper
label_desc: Use image node_id 1 for calling the SystemUttSelfBleu3MetricHandler obj's underlying member method code with example usage: obj.metric_key() and returns: str