[SPARK-14595][SQL] add input metrics for FileScanRDD

## What changes were proposed in this pull request?

This is roughly based on the input metrics logic in `SqlNewHadoopRDD`

## How was this patch tested?

Not sure how to write a test, I manually verified it in Spark UI.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #12352 from cloud-fan/metrics.