Allow use of both `@trace` and `@tag_args` stacked on the same function (#13453)

```py
@trace
@tag_args
async def get_oldest_event_ids_with_depth_in_room(...)
  ...
```

Before this PR, you would see a warning in the logs and the span was not exported:
```
2022-08-03 19:11:59,383 - synapse.logging.opentracing - 835 - ERROR - GET-0 - @trace may not have wrapped EventFederationWorkerStore.get_oldest_event_ids_with_depth_in_room correctly! The function is not async but returned a coroutine.
```