Resolve Environments#additional_metrics TypeError, ensure unix format

What does this MR do?

Verifying changes for https://gitlab.com/gitlab-org/gitlab-ce/issues/31368 on staging revealed a mistake in the Environment#additional_metrics endpoint. This MR makes the change and adds a happy-path spec, which was missing.

The feature still didn't work, however, because FE was passing a unix timestamp in milliseconds instead of seconds, per the unix spec. This MR also addresses that problem.

This MR should bring time windows for the metrics dashboard up to fully functional.

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry
• [n/a] 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

Closes #60162 (closed)