Return empty receivers list if tracing backend isn't connected

[mmkay]

Issue

When a tracing backend isn't connected, grafana-agent errors out due to a missing guard. Also, when the guard is added, the same issue as in canonical/grafana-agent-operator#219 appears.

Solution

Add the same guard as in the machine charm. Also return an empty receivers list if the upstream tracing backend isn't ready yet.

Context

Testing Instructions

Deploy the following bundle:

bundle: kubernetes
applications:
  grafana-agent-k8s:
    charm: local:grafana-agent-k8s-2
    scale: 1
    constraints: arch=amd64
    storage:
      data: kubernetes,1,1024M
  postgresql-k8s:
    charm: postgresql-k8s
    channel: 14/edge
    revision: 470
    resources:
      postgresql-image: 164
    scale: 1
    constraints: arch=amd64
    storage:
      pgdata: kubernetes,1,1024M
    trust: true
relations:
- - postgresql-k8s:tracing
  - grafana-agent-k8s:tracing-provider

Observe that neither of the charms errors out or shows pydantic validation errors once the relation settles. At this moment, postgres is expected to throw this error:

unit-postgresql-k8s-0: 15:33:21 ERROR unit.postgresql-k8s/0.juju-log no receiver found with protocol='otlp_http'

This should become a warning once canonical/tempo-coordinator-k8s-operator#95 is merged and the library gets updated.

Upgrade Notes