[9.2] Fix date range in DownsampleIT test (elastic#136930) (elastic#136934)

Manual backport of elastic#136930

Author: limotova

x-pack/plugin/downsample/src/internalClusterTest/java/org/elasticsearch/xpack/downsample/DownsampleIT.java

@@ -322,7 +322,7 @@ public void testPartialNullMetricsAfterDownsampling() throws Exception {
         bulkIndex(dataStreamName, sourceSupplier, 100);
         // And index documents with values
         sourceSupplier = () -> {
-            String ts = randomDateForRange(now.minusSeconds(60 * 14).toEpochMilli(), now.plusSeconds(60 * 30).toEpochMilli());
+            String ts = randomDateForRange(now.minusSeconds(60 * 14).toEpochMilli(), now.plusSeconds(60 * 29).toEpochMilli());
             try {
                 return XContentFactory.jsonBuilder()
                     .startObject()