Test fix: run test that requires a feature flag only in snapshot builds (elastic#139056)

Run tests that downsample exponential histograms only in snapshot builds

Author: gmarouli

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

@@ -7,6 +7,7 @@
 
 package org.elasticsearch.xpack.downsample;
 
+import org.elasticsearch.Build;
 import org.elasticsearch.action.admin.cluster.node.capabilities.NodesCapabilitiesRequest;
 import org.elasticsearch.action.admin.indices.delete.DeleteIndexRequest;
 import org.elasticsearch.action.admin.indices.delete.TransportDeleteIndexAction;
@@ -137,6 +138,8 @@ public void testAggregateMethod() throws Exception {
     }
 
     private void downsampleWithSamplingMethod(DownsampleConfig.SamplingMethod method) throws Exception {
+        // TODO: remove when FeatureFlag is removed and add minimum required version to yaml spec
+        assumeTrue("Only when exponential_histogram feature flag is enabled", Build.current().isSnapshot());
         String dataStreamName = "metrics-foo";
         String mapping = """
             {