move copy-to-parent into stop

this really just makes `start` and `stop` more symetric. It also means that it
behaves correctly if you manually `set_log_context` rather than using the
context manager.

synapse/logging/context.py

@@ -306,14 +306,6 @@ def __exit__(self, type, value, traceback) -> None:
                 logger.warning(
                     "Expected logging context %s but found %s", self, current
                 )
-        # if we have a parent, pass our CPU usage stats on
-        if self.parent_context is not None and hasattr(
-            self.parent_context, "_resource_usage"
-        ):
-            self.parent_context._resource_usage += self._resource_usage
-
-            # reset them in case we get entered again
-            self._resource_usage.reset()
 
     def copy_to(self, record) -> None:
         """Copy logging fields from this context to a log record or
@@ -363,6 +355,15 @@ def stop(self) -> None:
 
         self.usage_start = None
 
+        # if we have a parent, pass our CPU usage stats on
+        if self.parent_context is not None and hasattr(
+            self.parent_context, "_resource_usage"
+        ):
+            self.parent_context._resource_usage += self._resource_usage
+
+            # reset them in case we get entered again
+            self._resource_usage.reset()
+
     def get_resource_usage(self) -> ContextResourceUsage:
         """Get resources used by this logcontext so far.