WIP: Add per deployment sparkline metrics

Adds deployment metrics at path .../environements/:env_id/deployments/:deployment_iid/metrics.json

Notes for reviewer

I'm not entirely happy about this method signature metrics(environment, timeframe_start: start, timeframe_end: t_end) I'm very open to suggestions. I thought about creating Range like object that will hold both. But I can imagine situations where I don't want to give either start or end and expect to get metrics.

While this MR is in WIP state the back end portion of it has been foreseeably finished. And to streamline the process I'd like to review BE portion of it beforehand.

Todo:

• Initially review backend portion of this MR
• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)

What are the relevant issue numbers?

Related to: #26944 (closed)