run_poller: UNKNOWN:Timer list shutdown

I'm seeing these logs in stderr from docker hosts running some gRPC queries using this client:

```
E0921 17:38:25.652816051 244657 backup_poller.cc:124] run_poller: UNKNOWN:Timer list shutdown {created_time:"2022-09-21T17:38:25.652619086+00:00"}
E0921 19:14:08.230445287 322145 backup_poller.cc:124] run_poller: UNKNOWN:Timer list shutdown {created_time:"2022-09-21T19:14:08.230375719+00:00"}
E0921 19:23:53.008688653 341242 backup_poller.cc:124] run_poller: UNKNOWN:Timer list shutdown {created_time:"2022-09-21T19:23:53.008611811+00:00"}
```

Seems to come from - https://github.com/grpc/grpc/blob/master/src/core/ext/filters/client_channel/backup_poller.cc

Unfortunately, I haven't been able to find or capture any other related messages, and I haven't been able to reproduce it locally. I'm wondering if this is something anyone else has seen?

Start by reviewing gRPC's src/core/ext/filters/client_channel/backup_poller.cc around the reported log location and compare it with the Docker-host and gRPC-query conditions described here. Investigate whether the message indicates a client problem, ideally by finding related logs or a reproducible case; done means the cause and any required change are documented.

Written by the indexing model from the issue text.