docs: update _index.md, If the service_name label exists, it's not …

…overwritten (backport release-3.1.x) (#15173)

Co-authored-by: Cyrille Le Clerc <[email protected]>

docs/sources/setup/upgrade/_index.md

@@ -93,6 +93,7 @@ Loki 3.0 will automatically assign a `service_name` label to all ingested logs b
 
 Loki will attempt to create the `service_name` label by looking for the following labels in this order:
 
+  - service_name
   - service
   - app
   - application