Update observability alert query worker to return early if feature not enabled globally

• Add :observability to globally available features
• Update alert query worker to return early if feature is not present on instance

Related to gitlab-org/opstrace/opstrace#2974.

/cc @mappelman

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

Numbered steps to set up and validate the change are strongly suggested.

1. Setup GDK with alert management integration enabled on a project and with EE license.
2. The worker can be run on the console with: > Observability::AlertQueryWorker.new.perform.
3. While it is possible to test most of the functionality for alert creation via tests itself, additional GOB backend will be required to test end to end. See devvm.