Hardened "add" arithmetic for usage data

What does this MR do?

Hardened "add" arithemetic for usage data with SQL export feature.

Will enhance "template_repositories" metric SQL #322099 (closed) . We need + because union or SQL OR or SQL summation is slow. Next MR !54805 (merged)

Docs update !54804 (merged)

For SQL exporting

my_metric: count(Issue) + count(User)

We shall get:

SELECT (SELECT COUNT("projects"."id") FROM "projects" WHERE "projects"."namespace_id" IS NULL) + (SELECT COUNT("projects"."id") FROM "projects" INNER JOIN namespaces ON projects.namespace_id = namespaces.custom_project_templates_group_id);

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
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