BUG23567257: Fix _id auto-generation

On inserting a document into a collection without specifying the `_id`
attribute, the following error is thrown:

Data too long for column '_id' at row 1

UUID generates a 36 character long string but `_id` can only take a
maximum of 32 characters. Generating the UUID as a 32-character
hexadecimal string fixes the issue.

The existing tests for `Collection.add` will pass with this fix.

Author: amitab

lib/mysqlx/dbdoc.py

@@ -53,7 +53,7 @@ def keys(self):
 
     def ensure_id(self):
         if "_id" not in self.__dict__:
-            self.__dict__["_id"] = str(uuid.uuid4())
+            self.__dict__["_id"] = uuid.uuid4().hex
         return self.__dict__["_id"]
 
     def __str__(self):