Add project-level Traces/Metrics/Logs links to Monitor sidebar

What does this MR do and why?

We want users to be able to access Observability from the projects page

Behind the project_observability_menu_items feature flag, adds Traces, Metrics, and Logs nav links to the project Monitor sidebar pointing to the Projects::ObservabilityController routes. Please note the legacy EE-only items that point to the now-defunct tracing/metrics/logs controllers. Those haven't been removed yet.

The items are implemented in CE (not the EE extension) so they render for all users with the read_observability_portal permission on the project's group.

References

• Replace project Monitor sidebar Tracing/Metrics/Logs with observability controller links

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the flag for a project:

   Feature.enable(:project_observability_menu_items, Project.find(<id>))

2. Ensure the user has Developer+ access on the project's group
3. Navigate to the project → Monitor in the left sidebar
4. Verify Traces, Metrics, and Logs appear with URLs containing /observability/ and a compositeQuery param including the project ID
5. Disable the flag — verify the new items disappear

MR acceptance checklist

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