Update docs for Prometheus with metricName examples

[rickbrouwer]

In v2.12, the deprecated metricName is removed from the Prometheus scaler example.

However, there are still some examples in the keda docs where this appears as an example of a Prometheus trigger. These still need to be cleaned up.

[rickbrouwer]

@wozniakjan

Because I am no Repo Reviewer for the keda-docs, can you add labels 'first-good-issue' and 'help-wanted'.
Thanks.

[wozniakjan]

thanks @rickbrouwer! Labels are now in place

[turtlelovesshoes]

I am interested in helping with this!

[rickbrouwer]

Thanks @turtlelovesshoes !

[turtlelovesshoes]

@rickbrouwer I looked through the docs with not alot of expereince context and identified these files as the 'Prometheus Scaler Example'

• content/docs/<2.12 - 2.19>/scalers/prometheus.md
  and it looks like those versions of the file do not have the metricName anywhere in those files. Are there other file references I am missing?

[rickbrouwer]

It's included in several more documents, such as blog posts and for example deploy.md.
The purpose of this issue is to review all documentation in its entirety.