Require time window params in metrics dashboard

What does this MR do?

Related Issue:

https://gitlab.com/gitlab-org/gitlab-ce/issues/59313

While FE was being implemented for time windows in the metrics dashboard (a dropdown to specify a time range for which metrics should be fetched & displayed), it made sense to make start and end time parameters optional. Now that the FE has been merged, we should require these values on all requests.

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
• [n/a] Database guides
• [n/a] 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.
• [n/a] 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:

• [n/a] Label as security and @ mention @gitlab-com/gl-security/appsec
• [n/a] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [n/a] Security reports checked/validated by a reviewer from the AppSec team

Closes #59313 (closed)