fix: configure prometheus_remote_write config for non-leader

[ca-scribner]

Issue

This PR fixes this issue where grafana-agent is not correctly setting its prometheus_remote_write settings for non-leader units, leading to non-leader units not properly writing their data to prometheus

Solution

The issue is caused by leadership guards in the grafana-cloud-integrator library's GrafanaCloudConfigRequirer relation handling. Specifically, it does nothing on relation changed/relation broken events.

The solution here is:

• add tests that demonstrate the prometheus_remote_write settings are written to config for both leader and non-leader units
• bump the grafana-cloud-integrator lib to fix the issue

Context

Testing Instructions

juju add-model ga1
juju deploy ubuntu -n 2
juju deploy grafana-agent --channel=latest/edge
juju deploy grafana-cloud-integrator --channel latest/edge --config prometheus-url=http://some.domain.name:9090/api/v1/write
juju relate grafana-agent ubuntu
juju relate grafana-agent:grafana-cloud-config grafana-cloud-integrator:grafana-cloud-config

# Then compare the following:
juju ssh grafana-agent/0 -- cat /etc/grafana-agent.yaml | yq .integrations.prometheus_remote_write
juju ssh grafana-agent/1 -- cat /etc/grafana-agent.yaml | yq .integrations.prometheus_remote_write

where we should see both units having the same data:

- basic_auth:
    password: ''
    username: ''
  tls_config:
    insecure_skip_verify: false
  url: http://some.domain.name:9090/api/v1/write

Upgrade Notes

[sed-i]

I wonder how much of this test should reside in canonical/grafana-cloud-integrator.

[ca-scribner]

I think the only part that's testable in grafana-cloud-integrator is added already here. The rest of this test is grafana-agent business logic (confirm the settings appear in the place they need to be). Although tbh this unit test is a pretty large unit test, more an integration test that just doesn't need juju...