Update observability-metrics.md

[jmcx]

Added a warning for users installing Prometheus using the Operator.

[tzununbekov]

It is very subjective, but for me, the issue we're solving with this update is rather on the "special case" side, so maybe we could make this warning less intrusive. We can simply mention it in the config snippet with a short explanation, like this:

---

---

Explanation wording can be changed if you have something better in mind. Or we can uncomment those two lines, it shouldn't break anything. What do you think?

[jmcx]

Sure! I like it. I'll make that change and add the function examples too.

[jmcx]

Maybe we can merge this PR and do the function stuff in another one.