[HBASE-7276] bug in TestRegionServerMetrics.testNumReadsAndWrites

Author: adela

Summary: in https://phabricator.fb.com/D667725 we should update preNumReads and preNumWrite instead of updating preNumReads twice in TestRegionServerMetrics

Test Plan: TestRegionServerMetrics

Reviewers: liyintang, rshroff

Reviewed By: rshroff

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D685437

Task ID: 1981461

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1437638 13f79535-47bb-0310-9956-ffa450edef68