public MetricGroup group(String groupName, String... tagKeyValues) {
        MetricGroupId groupId = groupId(groupName, tagKeyValues);
        MetricGroup group = groupsByName.get(groupId);
        if (group == null) {
            group = new MetricGroup(groupId);
            MetricGroup previous = groupsByName.putIfAbsent(groupId, group);
            if (previous != null)
                group = previous;
        }
        return group;
    }
@Test
    public void testGettingGroupWithOddNumberOfTags() {
        assertThrows(IllegalArgumentException.class,
            () -> metrics.group("name", "k1", "v1", "k2", "v2", "extra"));
    }