Make run_token a property on Scope

Eventually this should just be a value.

python/cog/server/scope.py

@@ -6,6 +6,9 @@
 from ..types import ExperimentalFeatureWarning
 
 
+# This is a bit of a mess right now, Scope should just be a dictionary-like
+# object that has plain values.  Right now it's a ContextVar that wraps other
+# ContextVars; in future it should be a ContextVar that wraps a bag of values.
 class Scope:
     def __init__(
         self,
@@ -14,7 +17,11 @@ def __init__(
         run_token: Callable[[], Optional[str]],
     ) -> None:
         self.record_metric = record_metric
-        self.run_token = run_token
+        self._run_token = run_token
+
+    @property
+    def run_token(self) -> Optional[str]:
+        return self._run_token()
 
 
 _current_scope: ContextVar[Optional[Scope]] = ContextVar("scope", default=None)