This introduces a generic worker capability
`ApplicationWorker#log_extra_metadata_on_done(key, value)` to add
metadata that will later be logged with the done job payload. When
called this method will add the key/value to a `Hash` which is then
pushed all the way up the middleware to the `StructuredLogger` and
amended to the done log statement.

This MR also uses this functionality in ElasticIndexBulkCronWorker.

This is an improved way of doing the same thing as was implemented in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/30265 which allows
easily correlating the duration of a job with the size of the job being
processed. This is better because sidekiq done logs are already broken
out in detail for DB time, CPU time, Redis time and so we'd prefer to
build on that existing behaviour when doing our analysis.