Add Prometheus counters to subscribed ActionCable events

What does this MR do?

Adds ActionCable event subscriptions that hook into Prometheus metric counters for:

• transmit
• subscription successful
• subscription rejected
• broadcast

How to test:

1. Enable features: enable broadcast_issue_updates and real_time_issue_sidebar
2. Enable ActionCable in your gdk.yml - see https://docs.gitlab.com/omnibus/settings/actioncable.html. Runs as an endpoint, mounted at /cable. I found https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/master/doc/configuration.md#actioncable-settings helpful under the GDK.
3. Run gdk reconfigure and gdk restart (see gitlab-development-kit!1318 (comment 358143138)).
4. Log in to your local gdk in-browser
5. Go to any issue and change the assignee (this uses ActionCable)
6. To check metrics see !56157 (comment 526999525)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #296845 (closed)