@Test public void testUnregisterSource() {

  MetricsSystem ms = new MetricsSystemImpl();
  TestSource ts1 = new TestSource("ts1");
  TestSource ts2 = new TestSource("ts2");
  ms.register("ts1", "", ts1);
  ms.register("ts2", "", ts2);
  MetricsSource s1 = ms.getSource("ts1");
  assertNotNull(s1);
  // should work when metrics system is not started
  ms.unregisterSource("ts1");
  s1 = ms.getSource("ts1");
  assertNull(s1);
  MetricsSource s2 = ms.getSource("ts2");
  assertNotNull(s2);
  ms.shutdown();
}