Response from calling ReadRows may include row data, progress and
throttling information.
Serialized row data in AVRO format.
Number of serialized rows in the rows block. This value is
recorded here, in addition to the row_count values in the
output-specific messages in rows, so that code which needs
to record progress through the stream can do so in an output
format-independent way.
Throttling status. If unset, the latest response still
describes the current throttling status.
