The fraction of rows assigned to the stream that have been
processed by the server so far, not including the rows in
the current response message.
This value, along with at_response_end, can be used to
interpolate the progress made as the rows in the message are
being processed using the following formula:
at_response_start + (at_response_end - at_response_start) * rows_processed_from_response / rows_in_response.
Note that if a filter is provided, the at_response_end
value of the previous response may not necessarily be equal
to the at_response_start value of the current response.
Similar to at_response_start, except that this value
includes the rows in the current response.