Update setting up Prometheus docs

psschwei: originally opened in serving and moved here. updates needed for docs can be found in this comment: https://github.com/knative/docs/issues/5217#issuecomment-1247306521

/area monitoring

## What version of Knative?

`1.7.1` (installed with https://github.com/knative/serving/releases/download/knative-v1.7.1/serving-core.yaml)

## Expected Behavior

* install knative-serving
* port-fowarded one of them to debug on prometheus metrics
* pods report metrics described in https://knative.dev/docs/serving/observability/metrics/serving-metrics/

## Actual Behavior

```bash
kubectl port-forward service/activator-service -n knative-serving 9090:9090
Forwarding from 127.0.0.1:9090 -> 9090
Forwarding from [::1]:9090 -> 9090
Handling connection for 9090
E0914 16:42:51.809345 3007207 portforward.go:400] an error occurred forwarding 9090 -> 9090: error forwarding port 9090 to pod 4f48d8d6708f2f6cc464957b6718d2ec89088951c6430acfb0fb04857e595a94, uid : exit status 1: 2022/09/14 07:42:51 socat[13441] E connect(5, AF=2 127.0.0.1:9090, 16): Connection refused
```

I had to specify the `metrics.backend-destination` and `metrics.request-metrics-backend-destination` to get this working.

## Steps to Reproduce the Problem

* follow instructions on https://knative.dev/docs/install/yaml-install/serving/install-serving-with-yaml/#prerequisites
* port-forward one of the components with 9090 port exposed
* curl localhost:9090 (same error with curl localhost:9090/metrics)

```yaml
apiVersion: v1
kind: ConfigMap
metadata:
name: config-observability
namespace: knative-serving
labels:
app.kubernetes.io/name: knative-serving
app.kubernetes.io/component: observability
app.kubernetes.io/version: "1.7.1"
annotations:
knative.dev/example-checksum: "fed4756e"
data: {}
```

Start with the linked issue comment and the serving metrics page, then compare them with the YAML-install prerequisites. Update the Prometheus setup guidance to cover the reported port-forwarding and metrics-backend configuration, and verify that the documented steps match the Knative 1.7.1 behavior.

Written by the indexing model from the issue text.