@VisibleForTesting
    String buildIndexName(final int number) {
        return config.indexPrefix() + SEPARATOR + number;
    }
@Test
    public void testBuildIndexName() {
        assertEquals("graylog_0", mongoIndexSet.buildIndexName(0));
        assertEquals("graylog_1", mongoIndexSet.buildIndexName(1));
        assertEquals("graylog_9001", mongoIndexSet.buildIndexName(9001));
    }