Fix platform instance support on Druid ingestion

If the platform instance was specified in a Druid ingestion, it was always
set twice, because it was overrided in the get_identifier, and then added
again down the line.
This may break existing users that already rely on the broken
platform instance implementation, but they could still manually set
it to the old pattern in their dhub file.

docs/how/updating-datahub.md

@@ -26,6 +26,8 @@ This file documents any backwards-incompatible changes in DataHub and assists pe
 
 - #12671: The `priority` field of the Incident entity is changed from an integer to an enum. This field was previously completely unused in UI and API, so this change should not affect existing deployments.
 
+- #12716: Fix the `platform_instance` being added twice to the URN. If you want to have the previous behavior back, you need to add your platform_instance twice (i.e. `plat.plat`).
+
 
 ### Known Issues
 

metadata-ingestion/src/datahub/ingestion/source/sql/druid.py

@@ -50,11 +50,7 @@ def get_sql_alchemy_url(self):
     """
 
     def get_identifier(self, schema: str, table: str) -> str:
-        return (
-            f"{self.platform_instance}.{table}"
-            if self.platform_instance
-            else f"{table}"
-        )
+        return f"{table}"
 
 
 @platform_name("Druid")