Add Prometheus instructions and AM reference to Readme

[keturiosakys]

Description

This update adds a small para to the README explaining how to spin up Prometheus locally (with am)

Checklist

• [ ] The CHANGELOG is updated.
• [ ] The open-telemetry example in repository works fine:
  • the docker compose file is valid
  • the application compiles and run
  • alerts are getting triggered in Prometheus
• [ ] The prometheus example in repository works fine:
  • the docker compose file is valid
  • the application compiles and run
  • alerts are getting triggered in Prometheus
  • exemplars are accessible on the graph