clarify ports for GKE users

[domalessi]

What does this PR do?

Per this ticket, a customer on GKE experienced ~60 million Agent errors per month because the Datadog Agent was configured to scrape the default port (9090) instead of GKE's fixed port (9990). GKE does not allow users to change the port Cilium exposes metrics on, so the Agent must be configured accordingly.

I updated the Cilium integration documentation to clarify that the metrics port is environment-dependent, with specific guidance for GKE users:

• Added an info alert in the Installation section noting that GKE-managed Cilium uses port 9990 (fixed and not configurable)
• Expanded the Host Configuration note to list port requirements by environment
• Added a GKE-specific example in the Containerized section showing the correct agent_endpoint configuration

Motivation

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• 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

Review from drichards-87 is dismissed. Related teams and files:

• documentation
  • cilium/README.md

[github-actions]

⚠️ Recommendation: Add qa/skip-qa label

This PR does not modify any files shipped with the agent.

To help streamline the release process, please consider adding the qa/skip-qa label if these changes do not require QA testing.