Document different latency characteristic of query APIs compare to Prometheus

[kakkoyun]

Better to use Observability document and give examples over SLOs.

[metalmatze]

I'm still not sure what is missing exactly in the current docs. 🤔 What can we improve?

[brancz]

The SLOs set the targets, but there is no text explaining why, and it's especially confusing to those who came from a Prometheus background. We should just have an accompanying text that explains why.

[metalmatze]

Makes sense. I can add some comments on why they are that way.