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.
