This was unintentionally changed in
https://github.com/element-hq/synapse/pull/19068.
There is no real bug here. Without this PR, we just printed an empty
string for the `sentinel` logcontext whereas the prior art behavior was
to print `sentinel` which this PR restores.
Found while staring at the logs in
https://github.com/element-hq/synapse/issues/19165
### Reproduction strategy
1. Configure Synapse with
[logging](https://github.com/element-hq/synapse/blob/df802882bb2ec7d52d5c064c20531fe5a5b263b1/docs/sample_log_config.yaml)
1. Start Synapse: `poetry run synapse_homeserver --config-path
homeserver.yaml`
1. Notice the `asyncio - 64 - DEBUG - - Using selector: EpollSelector`
log line (notice empty string `- -`)
1. With this PR, the log line will be `asyncio - 64 - DEBUG - sentinel -
Using selector: EpollSelector` (notice `sentinel`)
Eric Eastwood
b9dda0ff22