Fix date range in DownsampleIT test (#136930)

Went outside the range whoopsy
Resolves #136914

Author: limotova

muted-tests.yml

@@ -564,9 +564,6 @@ tests:
 - class: org.elasticsearch.compute.aggregation.MedianAbsoluteDeviationIntAggregatorFunctionTests
   method: testSimpleSmallInput
   issue: https://github.com/elastic/elasticsearch/issues/136905
-- class: org.elasticsearch.xpack.downsample.DownsampleIT
-  method: testPartialNullMetricsAfterDownsampling
-  issue: https://github.com/elastic/elasticsearch/issues/136914
 - class: org.elasticsearch.xpack.esql.qa.mixed.FieldExtractorIT
   method: testTextFieldWithIpSubfieldMalformed {STORED}
   issue: https://github.com/elastic/elasticsearch/issues/136917

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

@@ -310,7 +310,7 @@ 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()