Consolidate "Observability" section into one how-to titled "Integrate with COS"

[NucciTheBoss]

Currently we have the Observability section split into multiple how-tos for integrating with COS, however, it flows a bit clunky since the how-tos are both short and tightly interconnected. We should instead just combine the section into one how-to, and the have additional reference documentation that outlines all the metrics that are collected from Charmed HPC.

This a common pattern that other charmed projects use for documenting how to integrate their project with COS: https://charmed-kubeflow.io/docs/integrate-with-cos