Add emails delivery metrics

What does this MR do?

This merge request adds email delivery Prometheus metrics.

It also moves existing Action Mailer hooks into the same module and initializer for better visibility and separation of concerns.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests added for this feature
• Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#49214