Render Observability UI iframe in sandbox

What does this MR do and why?

Issue - gitlab-org/opstrace/opstrace-ui#93 (closed)

• This MR includes sandbox attribute to observability UI iframe.

Screenshots or screen recordings

Follow up tasks

https://gitlab.com/gitlab-org/opstrace/opstrace-ui/-/issues/162

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

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.