Add ActionCable worker pool metrics

What does this MR do?

Follow-up for #217314 (closed)

This MR adds several new Prometheus metrics related to ActionCable, evolving around AC's internal thread pool:

Metric	Type	Description
action_cable_pool_min_size	Gauge	Minimum number of worker threads in ActionCable thread pool
action_cable_pool_max_size	Gauge	Maximum number of worker threads in ActionCable thread pool
action_cable_pool_current_size	Gauge	Current number of worker threads in ActionCable thread pool
action_cable_pool_largest_size	Gauge	Largest number of worker threads observed so far in ActionCable thread pool
action_cable_pool_pending_tasks	Gauge	Number of tasks waiting to be executed in ActionCable thread pool
action_cable_pool_tasks_total	Gauge	Total number of tasks executed by ActionCable thread pool

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Tested in Omnibus as well. ✅