Ensure default metrics embed is cached

What does this MR do?

This MR fixes a bug in the default metrics markdown embed, which kept it from actually being cached. This fixes both the issue and the false-positive in the spec.

Part of of series of MRs to cleanup & add support for embedding specific metrics in markdown!

Prior context: https://gitlab.com/gitlab-org/gitlab-ce/issues/30423 introduced a default embed.

The below MRs are all part of this work:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31006 - Move dashboard services to /services subdirectory & rename DynamicDashboardService to DefaultEmbedService
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31157 - Fix embeds caching bug
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31063 - Introduce support for fetching a specific embed
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31304 - Add support for dashboard params in GFM pipeline

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [n/a] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• 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:

• 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