Add Sidekiq middleware prometheus metrics

What does this MR do?

Document Sidekiq middleware Prometheus metrics

It adds these metrics in file lib/gitlab/sidekiq_middleware/server_metrics.rb:

          sidekiq_jobs_cpu_seconds:            ::Gitlab::Metrics.histogram(:sidekiq_jobs_cpu_seconds, 'Seconds of cpu time to run Sidekiq job', {}, SIDEKIQ_LATENCY_BUCKETS),
          sidekiq_jobs_completion_seconds:     ::Gitlab::Metrics.histogram(:sidekiq_jobs_completion_seconds, 'Seconds to complete Sidekiq job', {}, SIDEKIQ_LATENCY_BUCKETS),
          sidekiq_jobs_queue_duration_seconds: ::Gitlab::Metrics.histogram(:sidekiq_jobs_queue_duration_seconds, 'Duration in seconds that a Sidekiq job was queued before being executed', {}, SIDEKIQ_LATENCY_BUCKETS),
          sidekiq_jobs_failed_total:           ::Gitlab::Metrics.counter(:sidekiq_jobs_failed_total, 'Sidekiq jobs failed'),
          sidekiq_jobs_retried_total:          ::Gitlab::Metrics.counter(:sidekiq_jobs_retried_total, 'Sidekiq jobs retried'),
          sidekiq_running_jobs:                ::Gitlab::Metrics.gauge(:sidekiq_running_jobs, 'Number of Sidekiq jobs running', {}, :all),
          sidekiq_concurrency:                 ::Gitlab::Metrics.gauge(:sidekiq_concurrency, 'Maximum number of Sidekiq jobs', {}, :all)

Related issues

#33770 (closed)

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