Add Observability loading screen

What does this MR do and why?

Replace "Loading metrics..." text with a loading spinner. This is an update to match our design guidelines.

Duo generated summary

This change improves the loading experience for the observability feature by creating a dedicated loading component. Instead of showing just plain text that says "Loading metrics...", the app now displays a proper loading spinner with a centered layout and better visual styling. The loading message was also simplified from "Loading metrics..." to just "Loading..." to be more general-purpose. This makes the interface look more polished and professional when users are waiting for data to load.

References

Improve Observability iframe loading experience

Screenshots or screen recordings

Before	After
Screen_Recording_2025-10-30_at_10.46.52_AM	Screen_Recording_2025-10-30_at_10.46.01_AM

How to set up and validate locally

1. In rails console enable the feature flag and set the group settings

   group = Group.find_by_path('toolbox')
   Feature.enable(:observability_sass_features, group)
   group.create_observability_group_o11y_setting!(
      o11y_service_url: "http://localhost:8080",
      o11y_service_user_email: 'fakeemail@gitlab.com',
      o11y_service_password: "passwordmcpasswordface",
      o11y_service_post_message_encryption_key: "your-super-secret-encryption-key-here-32-chars-minimum",
   )

2. Get an instance of GitLab O11y running locally 3. Install Docker Desktop if you haven't already. 4. Checkout o11y and run it

git clone git@gitlab.com:gitlab-org/embody-team/experimental-observability/gitlab_o11y.git
cd gitlab_o11y
`O11Y_EMAIL='fakeemail@gitlab.com', O11Y_PASSWORD=passwordmcpasswordface ALLOWED_ORIGINS="http://localhost:3000" O11Y_NAME=O11y ORG_DISPLAY_NAME=GitLab docker compose -f deploy/docker/docker-compose.yaml up -d`

3. Load http://localhost:3000/groups/toolbox/-/observability/services and see the spinner

MR acceptance checklist

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