Backend for allowing sample metrics to be toggled from ui

What does this MR do?

Backend for #193171. This allows for a parameter sample_metrics=true to be passed to the backend, in which case the sample metrics endpoints will be rendered for the charts on the monitoring dashboard. Currently, the sample metrics are triggered by setting the env var USE_SAMPLE_METRICS, which prevents the sample metrics from easily being turned on and off.

Screenshots

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

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