Add progressive loader on Observability UI when embedded on GitLab

From UX point of view, we do not have any loader to indicate user that Observability-UI is loading, we might need to explore if we can add progressive loader like in issues page

https://gitlab.com/groups/gitlab-org/-/issues

We can also think about increasing timing for default loader of GOUI, which will fade away once GitLab UI is completely loaded.