code: def _measurements(session_group, metric_name):
    
    for session_index, session in enumerate(session_group.sessions):
      metric_value = _find_metric_value(session, metric_name)
      if not metric_value:
        continue
      yield _Measurement(metric_value, session_index)
docs: A generator for the values of the metric across the sessions in the group.
  
    Args:
      session_group: A SessionGroup protobuffer.
      metric_name: A MetricName protobuffer.
    Yields:
      The next metric value wrapped in a _Measurement instance.