Generate Prometheus sample metrics over pre-set intervals

What does this MR do?

This builds upon the previous MR !19987 (merged). Previously, we were only generating sample Prometheus metrics from 7 days ago until now, and so switching between the time intervals on the monitoring dashboard would yield identical results. This generates metrics over our 6 preset time intervals (30 minutes ago, 3 hours ago, 8 hours ago, 1 day ago, 3 days ago, 7 days ago) for a more "live" effect. The purpose of these MRs is to eventually allow us to quickly load up a local Gitlab environment with sample metrics data without having to configure and install Prometheus. This will be useful for demos, testing, and speeding up development when not directly working on the Prometheus integration.

This closes issue #36251 (closed).

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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

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
• [-] Security reports checked/validated by a reviewer from the AppSec team