feat(slack_alerting.py): refactor region outage alerting to do model based alerting instead

Unable to extract azure region from api base, makes sense to start with model alerting and then move to region

litellm/main.py

@@ -868,6 +868,7 @@ def completion(
             user=user,
             optional_params=optional_params,
             litellm_params=litellm_params,
+            custom_llm_provider=custom_llm_provider,
         )
         if mock_response:
             return mock_completion(