fix(NODE-6259): replace dynamically assigned length property with a static getter (mongodb#4173)

Author: sis0k0

src/bulk/common.ts

@@ -1178,6 +1178,10 @@ export abstract class BulkOperationBase {
     );
   }
 
+  get length(): number {
+    return this.s.currentIndex;
+  }
+
   get bsonOptions(): BSONSerializeOptions {
     return this.s.bsonOptions;
   }
@@ -1274,13 +1278,6 @@ export abstract class BulkOperationBase {
   }
 }
 
-Object.defineProperty(BulkOperationBase.prototype, 'length', {
-  enumerable: true,
-  get() {
-    return this.s.currentIndex;
-  }
-});
-
 function isInsertBatch(batch: Batch): boolean {
   return batch.batchType === BatchType.INSERT;
 }