Log result for ci_runners_stale_group_runners_prune_worker_cron

What does this MR do and why?

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This MR builds on !86477 (merged) to log the result of each job run as extra entries on the Kibana log (mentioned in this thread), as per https://docs.gitlab.com/ee/development/logging.html#logging-context-metadata-through-workers.

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Part of #361112 (closed)