Improve layout of dropdowns in the metrics dashboard page

What does this MR do?

This MR uses a gl-form-group component layout to wrap the dropdowns in the Prometheus / metrics page in order to improve the user experience on mobile (and desktop).

Before	After

Review app: https://gitlab-review-64730-metr-9vgrll.ce.gitlab-review.app/root/autodevops-deploy/environments/1/metrics

Responsiveness

3 Dropdowns Experience:

Dropdowns and buttons Experience: (Custom dashboard URL and Add metric button in EE):

• Add metric button is only available in EE but was mocked during development, its layout was considered.
• Stopped using some dashboard specific CSS, this CSS was removed.

Does this MR meet the acceptance criteria?

Conformity

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

Closes #64730 (closed)