costmap_2d observation_sources: 'expected_update_rate'?

edit

Answered my own question. It's really an "interval" rather than a "rate":

costmap_2d/src/observation_buffer.cpp:244:

bool ObservationBuffer::isCurrent() const
{
  if (expected_update_rate_ == ros::Duration(0.0))
    return true;

  bool current = (ros::Time::now() - last_updated_).toSec() <= expected_update_rate_.toSec();
  if (!current)
  {
    ROS_WARN(
        "The %s observation buffer has not been updated for %.2f seconds, and it should be updated every %.2f seconds.",
        topic_name_.c_str(), (ros::Time::now() - last_updated_).toSec(), expected_update_rate_.toSec());
  }
  return current;
}