Fix instances where report is not a string. (#1776)

Summary:
Pull Request resolved: #1776

This can happen sometimes. Use string formatting so it gets converted.

Reviewed By: colin2328

Differential Revision: D86540593

fbshipit-source-id: 42f3dc51d6674b8b4c71409d19e8c2db1047ac81

Author: zdevito

python/monarch/_src/actor/actor_mesh.py

@@ -274,7 +274,7 @@ def filter(self, record: Any) -> bool:
         fn = _monarch_actor().per_actor_logging_prefix
         ctx = _context.get(None)
         if ctx is not None and fn is not None:
-            record.msg = fn(ctx.actor_instance) + record.msg
+            record.msg = f"{fn(ctx.actor_instance)}{record.msg}"
         return True