Flaky test: LearnerMetricsTest.testLearnerMetricsTest()

Hi team,

New test testLearnerMetricsTest() added by the following commit failed 2 times on master with the same error:

junit.framework.AssertionFailedError: expected:<10> but was:<9>
at org.apache.zookeeper.server.quorum.LearnerMetricsTest.testLearnerMetricsTest(LearnerMetricsTest.java:88)
at org.apache.zookeeper.JUnit4ZKTestRunner$LoggedInvokeMethod.evaluate(JUnit4ZKTestRunner.java:80)

https://github.com/apache/zookeeper/pull/856
Submitted by: jhuan31

Maybe we should just blame the Thread.sleep(200) in line:77 and replace it with some clever logic.
Please take a look.

Regards,
Andor