Embed Gitlab Observability UI within GitLab

The plan for embedding GOUI within GitLab is as follow:

• Add new left sidebar nav item "Observability" at group level (UX proposal: gitlab-org/gitlab#371061 (closed))
• Button links to new ${groupId}/-/observability
• Embed iframe into above page, sourcing https://observe.gitlab.com/-/<groupId>
• Add Opstrace app url in frame-src and child-src CSP (https://gitlab.com/gitlab-org/gitlab/-/issues/369566#note_1053887449)
• The iframe should take up all space and collapse the sidebar, as mocked in the attached screenshot
• Gate feature behind Feature Flag
• (proposed from @mappelman ) Add darkmode and username query param (e.g. ?darkmode=<true|false>&username=<currently_logged_in_username_or_id>).
  • The first param will help us switch between dark/light mode instantly so the iFramed content matches the parent mode.
  • The second param we can use to invalidate sessions in Opstrace if the value changes - it doesn't matter what field we use (username or id). So when someone logs out of GitLab and logs in as a different user, they'll also be re-authenticated in Opstrace as the new user (because we'll invalidate the current session).
  • Moved to separate issues #81 (closed) #82 (closed)