[FLINK-39066][pipeline-connector][Fluss] Fix the metric histogram always return null (apache#4281)

Co-authored-by: Thorne <syyfffy@163.com>

Author: ThorneANN

flink-cdc-connect/flink-cdc-pipeline-connectors/flink-cdc-pipeline-connector-fluss/src/main/java/org/apache/flink/cdc/connectors/fluss/sink/v2/metrics/WrapperFlussHistogram.java

@@ -45,10 +45,7 @@ public long getCount() {
 
     @Override
     public HistogramStatistics getStatistics() {
-
-        flussHistogram.getStatistics();
-
-        return null;
+        return new FlinkHistogramStatistics(flussHistogram.getStatistics());
     }
 
     private static class FlinkHistogramStatistics extends HistogramStatistics {