chore: add clarifying comment for on_event envelope check

Author: shuoweil

packages/bigframes/bigframes/session/metrics.py

@@ -243,6 +243,7 @@ def on_event(self, envelope: Any):
         except ImportError:
             return
 
+        # Direct publisher.publish calls pass raw Event objects, while background query callbacks wrap events in an EventEnvelope to preserve progress bar context.
         if isinstance(envelope, bigframes.core.events.EventEnvelope):
             event = envelope.event
         else: