feat(ISSUE-1231): missing owner label metric and alert

[Zaggy21]

Description

This PR adds a resource-agnostic metric greenhouse_owned_by_label_missing with Prometheus alert.
It refactors all the metrics to be in the internal metrics package.

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation Update
• 🎨 Style
• 🧑‍💻 Code Refactor
• 🔥 Performance Improvements
• ✅ Test
• 🤖 Build
• 🔁 CI
• 📦 Chore (Release)
• ⏩ Revert

Related Tickets & Documents

• Closes [FEAT] - Missing owner labels metrics + alert #1231

Added tests?

• 👍 yes
• 🙅 no, because they aren't needed
• 🙋 no, because I need help
• Separate ticket for tests # (issue/pr)

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

Added to documentation?

• 📜 README.md
• 🤝 Documentation pages updated
• 🙅 no documentation needed
• (if applicable) generated OpenAPI docs for CRD changes

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes

[IvoGoman]

Good idea to unify the metrics related code in dedicated files across all controllers.

The metrics/plugin_metrics.go is a special case. It was created to update metrics in both helm and internal/controller/plugin packages.
There seems to be no reuse between controllers for the other files, so I would suggest to keep the metrics in a metrics.go file in the same package as the controller.

For the shared metrics, it might be good to combine them in a internal/util/metrics.go file. So we can keep metrics local to where they are being emitted and only move the shared functions out to the shared location.