Productivity Analytics: Add metric column component

What does this MR do?

This MR primarily fixes a bug in the MR table on the Productivity Analytics page when we pass null to n__. This leads to a Javascript error and prevents subsequent rows from being rendered.

It adds an isolated metric_column component, that checks if the passed value prop is a numeric value and renders a - otherwise.

It solves #3 of the issues listed in #31932 (closed)

This is behind the productivity_analytics feature flag.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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

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