Add aggregate test

Author: gmarouli

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

@@ -132,6 +132,10 @@ public void testLastValueMethod() throws Exception {
         downsampleWithSamplingMethod(DownsampleConfig.SamplingMethod.LAST_VALUE);
     }
 
+    public void testAggregateMethod() throws Exception {
+        downsampleWithSamplingMethod(DownsampleConfig.SamplingMethod.AGGREGATE);
+    }
+
     private void downsampleWithSamplingMethod(DownsampleConfig.SamplingMethod method) throws Exception {
         String dataStreamName = "metrics-foo";
         String mapping = """