Add not null constraints to prometheus_metrics table columns

What does this MR do?

Adds not null constraints to y_label and unit columns in prometheus_metrics table.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/66663

Screenshots

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
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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

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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
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