Create monitoring-mixin

[thedodd]

• [x] Create monitoring-mixin for Grafana dash, Alerts, &c
  • [x] Operator basics.
  • [x] Stream basics.
  • [x] Add metric to Stream for last compacted offset.
  • [x] Stream subscription templates with alerting for blocked sub (no registered consumers for a particular group).
  • [x] Pipeline templates with alerting for blocked pipelines (no registered consumers for a particular stage).
  • [x] All items should have Grafana alerting included.
• [x] Add docs to chart and guide on how to integrate the monitoring mixins into user's systems:
  • https://monitoring.mixins.dev/#how-to-use-mixins
  • https://github.com/grafana/helm-charts/tree/main/charts/grafana#sidecar-for-dashboards
• [x] Add docs to guide on monitoring, all of the prom metrics exposed &c.
• [x] Create PR to mixin repo to have this code added as. Moved to #119