feat: prometheus metrics

[yanksyoon]

Applicable spec:

Overview

• Adds prometheus metrics to the GitHub runner manager application
• Adds prometheus integration via COSAgent relation

** Note that the metrics were requested without refactoring which may lead to inconsistent or faulty metrics, due to the nature of the structure of the code. Any faulty metrics are NOT part of the responsibility of this PR.

Rationale

• To get observability into the charm/application.

Juju Events Changes

Module Changes

Library Changes

Checklist

• The charm style guide was applied.
• The contributing guide was applied.
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex).
• The changelog is updated with changes that affects the users of the charm.

[cbartz]

Thank you!

[github-actions]

Test results for commit 6ae9a0d

Test coverage for 6ae9a0d

Wrote XML report to coverage/coverage.xml

Static code analysis report

Run started:2025-07-09 09:11:30.819510

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 2055
  Total lines skipped (#nosec): 2
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 1

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):

[github-actions]

TICS Quality Gate

✔️ Passed

github-runner-operator

All conditions passed

See the results in the TICS Viewer

The following files have been checked for this project

• github-runner-manager/src/github_runner_manager/http_server.py
• github-runner-manager/src/github_runner_manager/manager/runner_manager.py
• github-runner-manager/src/github_runner_manager/manager/runner_scaler.py
• github-runner-manager/src/github_runner_manager/metrics/reconcile.py
• github-runner-manager/src/github_runner_manager/openstack_cloud/openstack_cloud.py
• github-runner-manager/tests/unit/openstack_cloud/test_openstack_cloud.py
• src/charm.py
• tests/integration/conftest.py
• tests/integration/test_prometheus_metrics.py

.github/workflows/tics.yaml / TICS / TICS GitHub Action

[cbartz]

We decided to remove the integration test test_runner_manager , as those tests are covered in other integration test and it prevented merging due to its flakiness.