Sidekiq client, in our case Rails, can also emit log messages related
to sidekiq jobs.

Instead of having these messages printed to STDOUT (which results in
`production.log`), this changes the Sidekiq.logger to a separate
writing to `log/sidekiq_client`. This allows us to ingest these
messages in the same index as the messages the sidekiq process
emits. Having them in the same index allows us to gather statistics
about jobs that haven't been processed.