Add prefix to template vars in url

Background

In the monitoring dashboards, template variables defined in the yml file are extracted and appended to the URL. These values are then rendered on the dashboard as input elements that the user can modify. The modified values are passed on to Prometheus API.

What does this MR do?

The user-defined variables from the yml file are appended to the URL as is #214533 (closed). This can cause collisions and break other features that extensively rely on certain URL params to uniquely identity chart panels.

This MR adds a var- prefix in order to differentiate between a template variable in the URL and other params.

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