SPARK-4178. Hadoop input metrics ignore bytes read in RecordReader insta...

...ntiation

Author: Sandy Ryza <sandy@cloudera.com>

Closes #3045 from sryza/sandy-spark-4178 and squashes the following commits:

8d2e70e [Sandy Ryza] Kostas's review feedback
e5b27c0 [Sandy Ryza] SPARK-4178. Hadoop input metrics ignore bytes read in RecordReader instantiation

(cherry picked from commit 28128150e7e0c2b7d1c483e67214bdaef59f7d75)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>