Add back the Prometheus example code

[MrAlias]

Blocked by the metric SDK having a working example in the new_sdk/main branch

Removed in #2802

[MrAlias]

@damemi are you still able to work on this? I'm happy to pick it up if not.

[damemi]

@MrAlias yup! opened a PR at #3168, basically just copying examples from some of the tests and setting up an endpoint. please take a look

[MrAlias]

Closed by #3168