docs: add prometheus + grafana deployment guide

[EyalPazz]

/fixes #747

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[EyalPazz]

@nirrozenbaum @danehans

[nirrozenbaum]

after running this step, I'm running the command from the helm output to get prometheus pod:

Get the Prometheus server URL by running these commands in the same shell:
  export POD_NAME=$(kubectl get pods --namespace monitoring -l "app.kubernetes.io/name=prometheus,app.kubernetes.io/instance=prometheus" -o jsonpath="{.items[0].metadata.name}")
  kubectl --namespace monitoring port-forward $POD_NAME 9090

when running the export POD_NAME=... I get the following error:

error: error executing jsonpath "{.items[0].metadata.name}": Error executing template: array index out of bounds: index 0, length 0. Printing more information for debugging the template:
	template was:
		{.items[0].metadata.name}
	object given to jsonpath engine was:
		map[string]interface {}{"apiVersion":"v1", "items":[]interface {}{}, "kind":"List", "metadata":map[string]interface {}{"resourceVersion":""}}

no pod matches the command. maybe something is missing from values.yaml?

[EyalPazz]

This is relying on the merge to main, it's currently pointing to the main branch despite it's not there yet

You can currently test using:
https://raw.githubusercontent.com/eyalpazz/gateway-api-inference-extension/add-prometheus-guide/config/manifests/prometheus/values.yaml

[nirrozenbaum]

this is the error after I copied values.yaml locally and used it

[nirrozenbaum]

do you mind trying the instructions on a fresh kind cluster?

[EyalPazz]

Of course, i'll try to do it later tonight