Use an image which exposes metrics in "Getting Started" tutorial

[creydr]

Currently the "Getting Started" tutorial, uses the hashicorp/http-echo which doesn't expose any metrics (like the later used http_requests_total) and relies on the metrics from the LoadBalancer. Users which go through this tutorial might not have to a LoadBalancer exposing those metrics, which results in the pods not getting scaled as expected.
This PR addresses it and uses an image (stefanprodan/podinfo), which exposes metrics (like the http_requests_total) which can get scraped by prometheus and thus make the scaling work via the http scaler work.

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[JorTurFer]

WDTY @kedacore/keda-core-contributors @kedacore/keda-core-maintainers ?

[creydr]

@zroubalik could you check on this?

[creydr]

@kedacore/keda-core-contributors @kedacore/keda-core-maintainers any update on this?

[rickbrouwer]

I looked at it from a slightly larger scope, namely the entire 'Getting Started with KEDA: A Simple Example'.

The first thing I notice is that it says:

"We'll use a basic Python HTTP server."

Is that true? Is that a Python HTTP server?

Further, since it's a simple example, I had someone else with no experience do the steps.
They stopped at This example assumes you have Prometheus installed in your cluster and scraping metrics from your application. I think if we want to keep it simple, we can leave it that way, but with a link to the steps for installing Prometheus. I think it would be very helpful if we also described that somewhere, so someone with (very) little experience gets the full experience. A reference elsewhere is enough for me.