[Follow-up] Use RssMemoryLimit monitor for Sidekiq

What does this MR do and why?

• @mkaeppler started a discussion: (+2 comments)

  Side note: the jobs mutex should never have been a public attribute; we should also address that 😬

• @mkaeppler started a discussion: (+1 comment)

  This seems unnecessarily complex. Can we hold this in a private attribute instead and initialize it in the initializer? We don't need strong_memoize then (why would this ever be nil anyway?) I think we have an unhealthy appetite for these kinds of lazy inits at GitLab. If we expect this resource to be used anyway, we should ready it in the class constructor. We should address this for EventReporter as well in the follow up

• This MR addresses jobs and job_mutex being publicly available outside of the Monitor class.
• It also initializes metrics for EventReporter instead of using strong_memoize

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