Embedding Grafana metrics panels in GFM - frontend

What does this MR do?

Adds the frontend for Grafana chart embeds in GFM.

Users may now paste a link of a Grafana dashboard chart (provided it meets certain conditions, outlined in the docs) into GitLab Flavored Markdown and it will unfurl into a live chart.

Note: this feature is currently behind a feature flag.

Frontend additions:

• Convert the metrics_dashboard endpoint call from blocking to polling.
• Map template variables within chart legend and tooltip labels.

Issue: #31376 (closed)

^ includes overview of other tasks and current status.

Backend MR: !18285 (merged)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry. To be added when FF is removed.
• Documentation created/updated or follow-up review issue created. Follow up here: !18541 (merged)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] Database guides
• [n/a] Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [n/a] Label as security and @ mention @gitlab-com/gl-security/appsec
• [n/a] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [n/a] Security reports checked/validated by a reviewer from the AppSec team