Logging formats for `influxd` and `influxd upgrade` don't match

`influxd` writes logs with format:
```
{"": ", ...}
```
`influxd upgrade` writes logs with format:
```
{"level": "", "ts": "", "msg": "", "": "", ...}
```

Our new DockerHub wrapper runs `upgrade` followed by plain `influxd` in the same container, and the logs get mixed together. The mismatch will be a pain for any downstream log-processing tools.

Start by tracing the logging entry points for `influxd` and `influxd upgrade`, comparing how each produces its documented output format. Identify the shared format that should be used, then verify that logs from both commands have matching structure and that existing logging checks still pass.

Written by the indexing model from the issue text.