Refactor DeploymentMetric to reuse code

What does this MR do?

Refactor DeploymentMetric to reuse code

Re-use shared Prometheus::Adapter code. Move that code out of app/services to lib/gitlab as it's generic PORO code, not a service.

Closes #119297 (closed)

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