Create a metric to report the size information (including total, min, med, max) like data size, spill size, etc.
Updates metrics based on the driver side value.
Updates metrics based on the driver side value. This is useful for certain metrics that are only updated on the driver, e.g. subquery execution time, or number of files.
A function that defines how we aggregate the final accumulator results among all tasks, and represent it in string for a SQL physical operator.