Remove "Add metric" button from custom dashboards

What does this MR do?

This MR removes the "Add metric" button from the monitoring dashboard in case the selected dashboard is not the default one.

The reason for this is that at the moment, clicking on "Add metric" adds the metric to the default dashboard. When the button is present in the context of other (custom) dashboards, this can mislead the users into believing they are adding a metric to the currently selected dashboard.

Related issue: #211648 (closed)

Before

Default dashboard Custom dashboard

After

Default dashboard Custom dashboard

Screenshots

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

Closes #211648 (closed)