code: func (c *Consumer) FilteredFirehose(
  	subscriptionId string,
  	authToken string,
  	filter EnvelopeFilter,
  ) (<-chan *events.Envelope, <-chan error) {
  	return c.firehose(newFirehose(
  		subscriptionId,
  		authToken,
  		WithEnvelopeFilter(filter),
  	))
  }
docs: // FilteredFirehose streams a filtered set of envelopes. It has functionality
  // similar to Firehose.