Add REDIS zcard (#14316)

### What problem does this PR solve?

As description.

### Type of change

- [x] Refactoring

rag/utils/redis_conn.py

@@ -284,6 +284,17 @@ class RedisDB:
             self.__open__()
         return 0
 
+    def zcard(self, key: str):
+        try:
+            res = self.REDIS.zcard(key)
+            return res
+        except Exception as e:
+            logging.warning(
+                f"RedisDB.zcard {key} got exception: {e}"
+            )
+            self.__open__()
+        return 0
+
     def incrby(self, key: str, increment: int):
         return self.REDIS.incrby(key, increment)