[HUDI-1836] Logging consuming instant to StreamReadOperator#processSplits (#2867)

hudi-flink/src/main/java/org/apache/hudi/source/StreamReadOperator.java

@@ -152,6 +152,9 @@ public class StreamReadOperator extends AbstractStreamOperator<RowData>
       return;
     }
 
+    // This log is important to indicate the consuming process, there is only one log message for one data bucket.
+    LOG.info("Processing input split : {}", split);
+
     format.open(split);
     try {
       RowData nextElement = null;

hudi-flink/src/main/java/org/apache/hudi/table/format/mor/MergeOnReadInputSplit.java

@@ -93,4 +93,19 @@ public class MergeOnReadInputSplit implements InputSplit {
   public int getSplitNumber() {
     return this.splitNum;
   }
+
+  @Override
+  public String toString() {
+    return "MergeOnReadInputSplit{"
+            + "splitNum=" + splitNum
+            + ", basePath=" + basePath
+            + ", logPaths=" + logPaths
+            + ", latestCommit='" + latestCommit + '\''
+            + ", tablePath='" + tablePath + '\''
+            + ", maxCompactionMemoryInBytes=" + maxCompactionMemoryInBytes
+            + ", mergeType='" + mergeType + '\''
+            + ", instantRange=" + instantRange
+            + '}';
+  }
+  
 }