fix: close batcher for writeToMultipleTables (#4182)

* fix: close batcher for writeToMultipleTables

* fix write

* move mutates.clear() to finally

Author: mutianf

bigtable-dataflow-parent/bigtable-hbase-beam/src/main/java/com/google/cloud/bigtable/beam/CloudBigtableIO.java

@@ -1003,15 +1003,18 @@ private BufferedMutator getMutator(Object context, String tableName) throws IOEx
 
     @FinishBundle
     public void finishBundle(FinishBundleContext c) throws Exception {
-      for (BufferedMutator bufferedMutator : mutators.values()) {
-        try {
-          bufferedMutator.flush();
-        } catch (RetriesExhaustedWithDetailsException exception) {
-          logExceptions(c, exception);
-          rethrowException(exception);
+      try {
+        for (BufferedMutator bufferedMutator : mutators.values()) {
+          try {
+            bufferedMutator.close();
+          } catch (RetriesExhaustedWithDetailsException exception) {
+            logExceptions(c, exception);
+            rethrowException(exception);
+          }
         }
+      } finally {
+        mutators.clear();
       }
-      mutators.clear();
     }
   }