Support concurrency limiting for mailer jobs

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The following discussion from !169220 (merged) should be addressed:

• @schin1 started a discussion:

  @dgruzd I notice this does not work for ActionMailer::MailDeliveryJob/ActiveJob::QueueAdapters::SidekiqAdapter::JobWrapper at the moment since it does not include ApplicationWorker. But it is not a problem since we do not define concurrency limits for mailers (nor am I sure how do we even do it 😅 since we do not define those worker classes).

  All Gitlab-defined Sidekiq workers would extend ApplicationWorker so we are good here.

  I'll resolve this thread by opening a follow-up issue in the meantime.

Currently, mailers do not have concurrency limits configured. It is not trivial to configure like other GitLab-defined sidekiq workers which include ApplicationWorker.

To support concurrency limit for mailers, we need to

• find an entrypoint to configure concurrency_limit -> { x } for the job wrapper
• support calling .concurrency_limit_resume method which is defined in ApplicationWorker

References:

• https://github.com/rails/rails/blob/v7.0.8.4/actionmailer/lib/action_mailer/mail_delivery_job.rb
• job wrapper: https://github.com/rails/rails/blob/v7.0.8.4/activejob/lib/active_job/queue_adapters/sidekiq_adapter.rb#L38