Add container-based config discovery support for gitlab_runner

[martavicentenavarro]

What does this PR do?

Adds container-based config discovery support to gitlab_runner, using a manual from_ports strategy targeting the default metrics port (9252).

• gitlab_url, prometheus_url, and allowed_metrics are now optional in the spec, since discovery can't derive them from a container.
• allowed_metrics defaults to [] instead of raising when omitted, so discovery-generated instances don't crash at runtime (covered by a new regression test).
• Clarified the allowed_metrics spec example, regenerated conf.yaml.example, and documented discovery prerequisites in the README.

Known gap: discovery can't derive gitlab_url, so the gitlab_runner.can_connect service check is never emitted for auto-discovered instances. test_e2e_discovery only asserts the Prometheus service check for this reason.

Motivation

https://datadoghq.atlassian.net/browse/DSCVR-521

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[martavicentenavarro]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: dd53213404

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[datadog-datadog-prod-us1-2]

 

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 94.27% (+5.91%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 99e0bec | Docs | Datadog PR Page | Give us feedback!}

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

Reviewed commit: 2b1b9439de

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[dd-octo-sts]

Validation Report

All 21 validations passed.

Show details

Validation	Description	Status
agent-reqs	Verify check versions match the Agent requirements file	✅
ci	Validate CI configuration and code coverage settings	✅
codeowners	Validate every integration has a CODEOWNERS entry	✅
config	Validate default configuration files against spec.yaml	✅
dep	Verify dependency pins are consistent and Agent-compatible	✅
http	Validate integrations use the HTTP wrapper correctly	✅
imports	Validate check imports do not use deprecated modules	✅
integration-style	Validate check code style conventions	✅
jmx-metrics	Validate JMX metrics definition files and config	✅
labeler	Validate PR labeler config matches integration directories	✅
legacy-signature	Validate no integration uses the legacy Agent check signature	✅
license-headers	Validate Python files have proper license headers	✅
licenses	Validate third-party license attribution list	✅
metadata	Validate metadata.csv metric definitions	✅
models	Validate configuration data models match spec.yaml	✅
openmetrics	Validate OpenMetrics integrations disable the metric limit	✅
package	Validate Python package metadata and naming	✅
qa-label	Validate the pull request declares whether it needs QA for the next Agent release	✅
readmes	Validate README files have required sections	✅
saved-views	Validate saved view JSON file structure and fields	✅
version	Validate version consistency between package and changelog	✅

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