Partitioned Agg: Add some counters.

Change-Id: I414dbe4bc0722fd5c6209033aa26d480460a0006
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4178
Reviewed-by: Ippokratis Pandis <ipandis@cloudera.com>
Tested-by: jenkins
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4207
Reviewed-by: Nong Li <nong@cloudera.com>