Cleanup policies: log the number of tags deleted

🐬 Context

Cleanup policies are used to keep the number of image tags in the Container Registry in check as image tags take physical space in the Container Registry.

Those policies are executed by background workers. Given the amount of work on gitlab.com, we implemented a number of limits around these executions, mainly using a limited capacity worker.

To have an eye on these background jobs, we created a Kibana Dashboard(internal).

This dashboard needs a few more metrics. Namely the number of tags deleted per job execution and the number of running jobs count.

🔨 What does this MR do?

• Add a cleanup_tags_service_before_delete_size and running_jobs_count in the extra metadata logged by the ContainerExpirationPolicies::CleanupContainerRepositoryWorker in the "done" message.
  • You can see those messages here
• No features are changed by this MR.

This being more a Stuff that should Just Work than anything, I didn't add a changelog entry.

🖼 Screenshots (strongly suggested)

n / a

💠 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

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