Refactor WorkersMap to WorkerAttributes

What does this MR do and why?

This MR copies methods in Gitlab::SidekiqMiddleware::ConcurrencyLimit::WorkersMap and move them into respective WorkerAttributes concern.

To derisk the changes, we added a feature flag sidekiq_concurrency_limit_middleware_v2 in the concurrency limit client/server middleware (which is the primary caller of the WorkersMap). The WorkersMap will then be removed once we remove the sidekiq_concurrency_limit_middleware_v2 feature flag too.

This also fixes:

• ConcurrencyLimitSampler not picking up workers with default concurrency limit
• ResumeWorker does not resume workers with default concurrency limit too.
• ResumeWorker does not respect the current limit of the worker (when throttled). This should have been implemented in !194627 (merged)

References

gitlab-com/gl-infra/data-access/durability/team#254 (closed)

How to set up and validate locally

ConcurrencyLimitSampler

1. Enable sidekiq exporter in GDK:

❯ cat gdk.yml | yq '.gitlab.rails_background_jobs'
sidekiq_exporter_enabled: true
sidekiq_exporter_port: 3807
sidekiq_health_check_enabled: false
sidekiq_health_check_port: 3907
sidekiq_queues:
  - default
  - mailers
timeout: 10
verbose: false

2. There should be hundreds of metrics reported in /metrics

❯ curl -s 'gdk.test:3807/metrics' | rg sidekiq_concurrency_limit_current_concurrent_jobs | wc -l
     950

MR acceptance checklist

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