docs: def _InitializeMetric(self, metadata):
      """See base class."""
      field_defs = stats_utils.FieldDefinitionTuplesFromProtos(
          metadata.fields_defs)
      if metadata.metric_type == rdf_stats.MetricMetadata.MetricType.COUNTER:
        self._counter_metrics[metadata.varname] = _CounterMetric(field_defs)
      elif metadata.metric_type == rdf_stats.MetricMetadata.MetricType.EVENT:
        self._event_metrics[metadata.varname] = _EventMetric(
            list(metadata.bins), field_defs)
      elif metadata.metric_type == rdf_stats.MetricMetadata.MetricType.GAUGE:
        value_type = stats_utils.PythonTypeFromMetricValueType(
            metadata.value_type)
        self._gauge_metrics[metadata.varname] = _GaugeMetric(
            value_type, field_defs)
      else:
        raise ValueError("Unknown metric type: %s." % metadata.metric_type)