disabling and re-enabling observability plugin results in terraform error

[Samuel Allan]

On 2023.2/edge, I disabled the observability plugin, then enabled it again:

```
sunbeam disable observability
sunbeam enable observability
```

This resulted in an error from terraform:

```
Error: Client Error

  with juju_integration.grafana-agent-to-receive-remote-write[0],
  on main.tf line 1037, in resource "juju_integration" "grafana-agent-to-receive-remote-write":
1037: resource "juju_integration" "grafana-agent-to-receive-remote-write" {

Unable to consume remote offer, got error: saas application called
"prometheus-receive-remote-write" from a different model already exists

Error: Client Error

  with juju_integration.grafana-agent-to-logging[0],
  on main.tf line 1051, in resource "juju_integration" "grafana-agent-to-logging":
1051: resource "juju_integration" "grafana-agent-to-logging" {

Unable to consume remote offer, got error: saas application called
"loki-logging" from a different model already exists

Error: Client Error

  with juju_integration.grafana-agent-to-cos-grafana[0],
  on main.tf line 1065, in resource "juju_integration" "grafana-agent-to-cos-grafana":
1065: resource "juju_integration" "grafana-agent-to-cos-grafana" {

Unable to consume remote offer, got error: saas application called
"grafana-dashboards" from a different model already exists

Error: Command '['/snap/openstack/479/bin/terraform', 'apply', '-auto-approve', '-no-color']' returned non-zero exit status 1.
```

It seems the consumer side of the cross model relations from observability model aren't cleaned up on disabling the plugin.

[Samuel Allan (samuelallan)]

A workaround is to manually clean up the offers before re-enabling observability:

```
juju remove-saas -m openstack prometheus-receive-remote-write
juju remove-saas -m openstack grafana-dashboards
juju remove-saas -m openstack loki-logging
juju remove-saas -m admin/controller prometheus-receive-remote-write
juju remove-saas -m admin/controller grafana-dashboards
juju remove-saas -m admin/controller loki-logging
```