Fix Ruby 2.7 kwarg warnings for Prometheus specs

What does this MR do?

This MR fixes:

spec/lib/gitlab/prometheus/query_variables_spec.rb:12: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call", "spec/lib/gitlab/prometheus/query_variables_spec.rb:12

Part of #257438 (closed)

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

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• [-] 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