feat: add prometheus integration

[dariofaccin]

Description

The upstream UDM added a metrics endpoint in v1.4.1. This PR adds prometheus_scrape integration to the K8s charm allowing the UDM to be scraped by Prometheus in the Juju ecosystem.

Usage

juju deploy sdcore-udm-k8s
juju deploy prometheus-k8s
juju integrate prometheus-k8s:metrics-endpoint sdcore-udm-k8s:metrics-endpoint

Reference

• feat: expose metrics for prometheus to scrape omec-project/udm#102

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library