fix(k8s): forward dashboards

[simskij]

resolves #240 resolves #87

Issue

currently, dashboards are only forwarded for the machine flavor of grafana agent

Solution

forward dashboards for k8s as well

Context

Testing Instructions

1. deploy cos lite
2. deploy this bundle, with a local build of the charm:

   bundle: pr-241
   saas:
   grafana:
   url: uk8s:admin/cos.grafana
   prometheus:
   url: uk8s:admin/cos.prometheus
   applications:
   ga:
   charm: local:grafana-agent-k8s-4
   channel: edge
   resources:
     agent-image: 30
   scale: 1
   pgsql:
   charm: postgresql-k8s
   channel: 14/edge
   revision: 128
   resources:
     postgresql-image: 116
   scale: 1
   relations:
   - - ga:metrics-endpoint
   - pgsql:metrics-endpoint
   - - ga:logging-provider
   - pgsql:logging
   - - ga:send-remote-write
   - prometheus:receive-remote-write
   - - ga:grafana-dashboards-provider
   - grafana:grafana-dashboard
   - - ga:grafana-dashboards-consumer
   - pgsql:grafana-dashboard

3. look in grafana and make sure you have all the dashboards

Release Notes

Forward dashboards on K8s.

[simskij]

As someone who's been back a month and barely remembers what Python is, this change is hard to review without any tests to show that it works. Any context I'm missing justifying the absence of tests?

tests added 👍🏼