Add metrics for number of spilled queries and total spilled bytes written.

Change-Id: Icdd06b6c2310d126278ad4c2e9f8280b4835d6f4
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4384
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: Nong Li <nong@cloudera.com>