print sampled block count under ENABLE_DEBUG_LOG compile mode

src/storage/access/ob_block_sample_iterator.h

@@ -166,6 +166,7 @@ private:
   const ObITableReadInfo *read_info_;
   ObMultipleScanMerge *scan_merge_;
   int64_t block_num_;
+  int64_t sample_block_cnt_;
   common::ObArenaAllocator range_allocator_;
   ObBlockSampleRangeIterator range_iterator_;
   blocksstable::ObDatumRange micro_range_;