Add Sidekiq memory killer prometheus metrics

What does this MR do?

For #32136 (closed).

Expose current_rss, soft_limit_rss and hard_limit_rss as part of Prometheus metrics.

Also expose the phase in which this memory killer is in, namely: checking_rss, stop_fetching_new_jobs, shutting_down, killing_sidekiq. Since prometheus does not support string as metrics value, we define phase as:

PHASE = { running: 1, above_soft_limit: 2, stop_fetching_new_jobs: 3, shutting_down: 4, killing_sidekiq: 5 }.freeze
... ...

# initialise phase metric:
sidekiq_memory_killer_phase:  ::Gitlab::Metrics.gauge(:sidekiq_memory_killer_phase, 'Current phase of Sidekiq Worker')