Adding Skeleton loader into Observability UI iframe

What does this MR do and why?

gitlab-org/opstrace/opstrace-ui#150 (closed)

• Add skeleton while observability UI is loading.
• If Observability UI is loading pretty fast (This is true mostly for local) DO not show skeletons.
• Once we show skeleton, If observability UI is loaded immediately keep the skeleton for sometime to avoid flashing effect.

Screenshots or screen recordings

Video Recording

gitlab-org/opstrace/opstrace-ui#142 (closed) (comment 1194290884)

How to set up and validate locally

1. This requires running Observability UI locally. Detailed steps for running it with gdk are here.
2. Once, gdk and Observability UI both are running, Go to a group page, e.g. http://localhost:3000/gitlab-org
3. Access the Observability tab from the left navbar. There will be three submenus Dashboards, Explore and Manage.
4. Visit page http://localhost:3000/groups/gitlab-org/-/observability/dashboards

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MR acceptance checklist

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