Support time window parameters in additional metrics endpoint

What does this MR do?

Adds support for specifying a time window on the environment additional_metrics endpoint. The start and end parameters match the format specified by the prometheus api, and are passed directly on to the prometheus client. This will facilitate a smoother transition for when we build out our prometheus endpoint. This work should enable the front end to specify a time range for the metrics dashboard.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/31368

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer