[SPARK-20151][SQL] Account for partition pruning in scan metadataTime metrics

## What changes were proposed in this pull request?
After SPARK-20136, we report metadata timing metrics in scan operator. However, that timing metric doesn't include one of the most important part of metadata, which is partition pruning. This patch adds that time measurement to the scan metrics.

## How was this patch tested?
N/A - I tried adding a test in SQLMetricsSuite but it was extremely convoluted to the point that I'm not sure if this is worth it.

Author: Reynold Xin <rxin@databricks.com>

Closes #17476 from rxin/SPARK-20151.