added scylla prom example

[hecomp]

Description

What does this PR do?

PR checklist for examples

Check these items before merging the PR.

• Each example contains a README file.
• README documents all steps and variables required to run the example. The simplest way to confirm this is to follow the instructions on a clean machine.
• README file contains the links to the official documentation with relevant configuration and to the metrics published if that is available.
• Used images of apps and collectors have to be pinned, "latest" should never be used.
• Evidences with screenshots provided from the reviewer.
• Includes a file called metrics.csv with metrics produced by the example.
  The file should have these 5 headings: Name, Description, Unit, DataType, Attributes.
  Description is not provided for all metrics, so it may be blank. Attributes are also not always provided. When there are multiple attributes on a metric reord they should be space separated.