config-reloader Restarts Multiple Times on Boot

I've been investigating why the `config-reloader` pod occasionally restarts multiple times while booting, and I've narrowed it down to [this line](https://github.com/GoogleCloudPlatform/prometheus-engine/blob/3d1c0fbf4211bd34b955079297bece2e738da3c0/cmd/config-reloader/main.go#L91). I'm seeing the following in the logs:

```
Get \"http://localhost:19090/-/ready\": dial tcp 127.0.0.1:19090: connect: connection refused
```

Per the comment on line 73, it seems like the intent would be to continue polling in this situation, but instead the process exits and has to be restarted.

Start in cmd/config-reloader/main.go, especially the polling comment around line 73 and the behavior at line 91. Reproduce the startup case with the readiness endpoint refusing connections, then run the relevant config-reloader tests or add coverage for that condition. Done means the process continues polling instead of exiting when localhost:19090 is not ready.

Written by the indexing model from the issue text.