Add observability to scheduled processing job

[Alek96]

After GH-172 introduced adaptive polling, the outbox processing job was changed from annotation-based scheduling (@Scheduled) to programmatic scheduling via taskScheduler.schedule(...).

Because of that change, the processing job is no longer observed in the same way Spring provides for @Scheduled methods. We should restore observability for this job by instrumenting it with ScheduledTaskObservationDocumentation.

[rolandbeisel]

@Alek96 Thank you for the contribution!