[Transform] enhance geobounds test for sparse data case (#72023)

use sparse data for geobounds agg, verifies the fix of #71874, adding debug
logging of index requests send by transform

Author: Hendrik Muhs

x-pack/plugin/transform/qa/single-node-tests/src/javaRestTest/java/org/elasticsearch/xpack/transform/integration/TransformPivotRestIT.java

@@ -1120,7 +1120,12 @@ public void testPivotWithBucketScriptAgg() throws Exception {
     public void testPivotWithGeoBoundsAgg() throws Exception {
         String transformId = "geo_bounds_pivot";
         String transformIndex = "geo_bounds_pivot_reviews";
-        setupDataAccessRole(DATA_ACCESS_ROLE, REVIEWS_INDEX_NAME, transformIndex);
+        String indexName = "reviews_geo_bounds";
+
+        // gh#71874 regression test: create some sparse data
+        createReviewsIndex(indexName, 1000, "date", false, 5, "location");
+
+        setupDataAccessRole(DATA_ACCESS_ROLE, indexName, transformIndex);
 
         final Request createTransformRequest = createRequestWithAuth(
             "PUT",
@@ -1130,7 +1135,7 @@ public void testPivotWithGeoBoundsAgg() throws Exception {
 
         String config = "{"
             + " \"source\": {\"index\":\""
-            + REVIEWS_INDEX_NAME
+            + indexName
             + "\"},"
             + " \"dest\": {\"index\":\""
             + transformIndex

x-pack/plugin/transform/src/main/java/org/elasticsearch/xpack/transform/transforms/TransformIndexer.java

@@ -896,6 +896,18 @@ private IterationResult<TransformIndexerPosition> processBuckets(final SearchRes
         );
 
         List<IndexRequest> indexRequests = indexRequestStream.collect(Collectors.toList());
+        if (logger.isDebugEnabled()) {
+            if (indexRequests.isEmpty()) {
+                logger.debug("[{}] processed buckets, nothing to be indexed", getJobId());
+            } else {
+                logger.debug(
+                    "[{}] processed buckets and created [{}] documents to be indexed, 1st document: [{}]",
+                    getJobId(),
+                    indexRequests.size(),
+                    indexRequests.get(0)
+                );
+            }
+        }
         IterationResult<TransformIndexerPosition> result = new IterationResult<>(indexRequests, newPosition, indexRequests.isEmpty());
 
         // NOTE: progress is also mutated in onFinish