Implement best practices for grafana-agent app

[marieroque]

Coming from here.

The goal of this issue is to have the App eventually adhere to standard cloud-native best-practices. The points below are to be considered non-exhaustive. Feel free to add some that make sense and remove some that don't. If this issue does not make sense (e.g. is a duplicate or covers a deprecated app) please close it.

Expected outcome:

• the app has properly set PDBs (as percentages)
• the app can scale horizontally based on Custom Metrics (if it makes sense)
• the app runs with more than 1 replica by default (if it needs HA)
• the app does not use emptyDir volumes if not strictly necessary

[Rotfuks]

This is not really feasible for us and will be closed out of scope for now. 

we are adressing any stability issues of the grafana agent app by improving the resource usage. Closing it for now.