class_docstr: The stream time metric.
  This metric only works at eval time.
  
  After the entire evaluation stream,
  this plugin metric emits the average time of that stream.
class_docstr_tok: ["The","stream","time","metric",".","This","metric","only","works","at","eval","time",".","After","the","entire","evaluation","stream",",","this","plugin","metric","emits","the","average","time","of","that","stream","."]
code: def before_eval(self, strategy: "SupervisedTemplate"):
      super().before_eval(strategy)
      self._time.update()
  
imports: import time
  from typing import TYPE_CHECKING, List
  from avalanche.evaluation import Metric, PluginMetric, GenericPluginMetric
  from avalanche.evaluation.metric_results import MetricValue, MetricResult
  from avalanche.evaluation.metric_utils import get_metric_name
  from avalanche.evaluation.metrics.mean import Mean
label_desc: Use image node_id 2 for calling the StreamTime obj's underlying member method code with example usage: obj.before_eval(strategy) without return types