Always record worker_id in @FLUENT_LOG

**Problem**

Fluentd is very incosintent about metadata annotation for `@FLUENT_LOG`.
Sometimes it shows the worker's ID as part of log records, and sometimes it won't:

```
2022-10-17 09:59:13.754376201 +0900 fluent.info: {"pid":75192,"ppid":75186,"worker":0,"message":"starting fluentd worker pid=75192 ppid=75186 worker=0"}
2022-10-17 09:59:13.754640834 +0900 fluent.info: {"worker":0,"message":"fluentd worker is now running worker=0"}
2022-10-17 09:59:16.736660074 +0900 fluent.debug: {"message":"fluentd main process get SIGINT"}
2022-10-17 09:59:17.263191871 +0900 fluent.debug: {"message":"fluentd main process get SIGTERM"}
2022-10-17 09:59:17.263328204 +0900 fluent.debug: {"message":"getting start to shutdown main process"}
```

This makes the monitoring unnecessarily hard. For example, we sometimes cannot
decide which worker emitted a paritcular error.

**How to reproduce the issue**

* Prepare the following configuration:
```xml


@type stdout


```
* Run Fluentd with `fluentd -c fluentd.conf -v`
* Check the log lines coming from `@FLUENT_LOG`.

**Desired Behavior**

* Fluentd should always make it clear which message comes from which worker.