Add the job size to start message logging

What does this MR do?

This would allow us to see how big jobs are in the log, perhaps allowing us to set a sensible limit.

The size gets calculated to log in the start message, which will also make it available in the done message.

This is triggered by gitlab-com/gl-infra/scalability#825

Screenshots (strongly suggested)

2021-02-03_15:07:22.02279 rails-background-jobs : {"severity":"INFO","time":"2021-02-03T15:07:22.022Z","queue":"cronjob:geo_sidekiq_cron_config","args":[],"class":"Geo::SidekiqCronConfigWorker","retry":0,"backtrace":true,"version":0,"queue_namespace":"cronjob","jid":"18ae604606589915c46cf449","created_at":"2021-02-03T15:07:20.460Z","meta.caller_id":"Cronjob","correlation_id":"fd398b879a658261a1d66546f81cb154","uber-trace-id":"d3bcfadcf8867b6e:d3bcfadcf8867b6e:0:1","enqueued_at":"2021-02-03T15:07:20.462Z","pid":9411,"message":"Geo::SidekiqCronConfigWorker JID-18ae604606589915c46cf449: done: 1.556568 sec","job_status":"done","scheduling_latency_s":0.003492,"job_size_bytes":395,"db_count":0,"db_write_count":0,"db_cached_count":0,"duration_s":1.556568,"cpu_s":0.387845,"completed_at":"2021-02-03T15:07:22.022Z","db_duration_s":0.011818}
2021-02-03_15:07:22.03309 rails-background-jobs : {"severity":"INFO","time":"2021-02-03T15:07:22.032Z","queue":"cronjob:schedule_merge_request_cleanup_refs","args":[],"class":"ScheduleMergeRequestCleanupRefsWorker","retry":0,"backtrace":true,"version":0,"queue_namespace":"cronjob","jid":"aec741c7f090ab272a528225","created_at":"2021-02-03T15:07:20.348Z","meta.caller_id":"Cronjob","correlation_id":"2439792b80e8e30c573fcfe3ade692e4","uber-trace-id":"81c02e57ef93738a:81c02e57ef93738a:0:1","enqueued_at":"2021-02-03T15:07:20.350Z","pid":9411,"message":"ScheduleMergeRequestCleanupRefsWorker JID-aec741c7f090ab272a528225: done: 1.680147 sec","job_status":"done","scheduling_latency_s":0.002115,"job_size_bytes":416,"db_count":1,"db_write_count":0,"db_cached_count":0,"duration_s":1.680147,"cpu_s":0.328562,"completed_at":"2021-02-03T15:07:22.032Z","db_duration_s":0.014764}

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• [-] Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done