Try to_dict method when encoding JSON (#799)

logfire/_internal/json_encoder.py

@@ -275,6 +275,13 @@ def to_json_value(o: Any, seen: set[int]) -> JsonValue:
 
         if isinstance(o, Sequence):
             return [to_json_value(item, seen) for item in o]  # type: ignore
+
+        try:
+            # Some VertexAI classes have this method. They have no common base class.
+            # Seems like a sensible thing to try in general.
+            return to_json_value(o.to_dict(), seen)
+        except Exception:  # currently redundant, but future-proof
+            pass
     except Exception:  # pragma: no cover
         pass