[HBASE-7276] Fix TestRegionServerMetrics.testNumReadsAndWrites

Author: adela

Summary:
There is only one RS in the mini cluster in the test and the
meta and root regions are on it. Because of that the calculations for
numReads and Writes sometimes get messed up. Added skipping the
calculations for this regions

Test Plan: test passing

Reviewers: liyintang

Reviewed By: liyintang

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D683273

Task ID: 1981461

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1435644 13f79535-47bb-0310-9956-ffa450edef68