Update Grafana proxy api time range params

What does this MR do?

!26333 (merged) updated the name of the params used to designate timestamps sent to prometheus. Our Grafana integration also proxies Prometheus, so it should be included in the name change.

This updates our Grafana proxy controller to accept start_time and end_time params, as well as adds assertions to all metrics embed cases to protect against unintentional API changes in future.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - Fixes a 12.10 regression
• [-] 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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
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