## What changes were proposed in this pull request?
It is not super intuitive how to update SQLMetric on the driver side. This patch introduces a new SQLMetrics.postDriverMetricUpdates function to do that, and adds documentation to make it more obvious.

## How was this patch tested?
Updated a test case to use this method.

Author: Reynold Xin <rxin@databricks.com>

Closes #17464 from rxin/SPARK-20134.