Show multimetric embeds on a single chart

What does this MR do?

Embeds with multiple metrics display correctly on the metrics dashboard, but when they are embedded, the are shown on separate charts. This MR fixes that so embeds match the dashboard.

Issue: #207459 (closed)

Testing!

1. Setup prometheus metrics in a project
2. Add multi-metric queries. See this keen rake task: !28490 (merged)
3. From the metrics dashboard (project > Operations > Metrics), scroll down to see the chart & copy the link to the chart from the options menu on the panel
4. Paste the url into a new GFM field in the project & save/preview to see the chart. It should match the one seen on the metrics dashboard.

Screenshots

UI	/metrics_dashboard response

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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:

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