Feature Request: Allow relativeCreated to be formatted as seconds instead of milliseconds

Python LogRecord has the attribute[ `relativeCreated`](https://docs.python.org/3/library/logging.html#logrecord-attributes)

> Time in milliseconds when the LogRecord was created, relative to the time the logging module was loaded.

It's currently possible to use FluentRecordFormatter to include that in the fluent events

```
version: 1
formatters:
fluent_fmt:
'()': fluent.handler.FluentRecordFormatter
format:
"host.hostname": '%(hostname)s'
"host.id": '%(hostname)s'
"host.uptime": '%(relativeCreated)d'
"log.logger": '%(name)s'
"log.level": '%(levelname)s'
"log.origin.file.name": '%(pathname)s'
"log.origin.file.line": '%(lineno)d'
"log.origin.function": '%(funcName)s'
```

but relativeCreated is in milliseconds, it would be very convenient to be able to convert that to seconds right in FluentRecordFormatter hence this feature request, please considering allowing something like

```
"host.uptime": '%(relativeCreatedInSeconds)d'
```

were the `relativeCreatedInSeconds` would be something added by FluentRecordFormatter.