Expose Sidekiq memory killer metrics to Prometheus

The following discussion from !16900 (merged) should be addressed:

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

  We should expose current_rss, soft_limit_rss and hard_limit_rss as part of Prometheus metrics.

  We should also expose the phase in which this memory killer is in, like that we are preparing for shutdown, etc.

  This should be a follow-up MR.