@Test(expectedExceptions = IllegalStateException.class)
  public void testMergingUnequalString() {

    final TestMergeableMetric metric1 = new TestMergeableMetric(),
        metric2 = new TestMergeableMetric();
    metric1.mustBeEqualString = "goodbye";

    Assert.assertFalse(metric1.canMerge(metric2));
    metric1.merge(metric2);
  }