Add itests to check for data persistence

[Abuelodelanada]

Issue

This PR add itests to:

• Check if prometheus data persists across unit restarts (pod churns / kubectl delete pod)
• Check if prometheus data persists across charm upgrade (juju refresh)
• Check if prometheus data persists when scaling down to zero units and scaling up again.

Testing Instructions

Run itests

[github-actions[bot]]

Libraries are not up to date with their remote counterparts. If this was not intentional, run charmcraft fetch-libs and commit the updated libs to your PR branch.

stdout

``` Library charms.alertmanager_k8s.v0.alertmanager_dispatch was already up to date in version 0.4. Library charms.grafana_k8s.v0.grafana_source was already up to date in version 0.10. Library charms.observability_libs.v0.kubernetes_service_patch was already up to date in version 0.6. Library charms.prometheus_k8s.v0.prometheus_remote_write was already up to date in version 0.5. Library charms.prometheus_k8s.v0.prometheus_scrape was already up to date in version 0.20. Library charms.traefik_k8s.v0.ingress_per_unit updated to version 0.10. ```

stderr

``` ```

[github-actions[bot]]

Libraries are not up to date with their remote counterparts. If this was not intentional, run charmcraft fetch-libs and commit the updated libs to your PR branch.

stdout

``` Library charms.alertmanager_k8s.v0.alertmanager_dispatch was already up to date in version 0.4. Library charms.grafana_k8s.v0.grafana_source was already up to date in version 0.10. Library charms.observability_libs.v0.kubernetes_service_patch was already up to date in version 0.6. Library charms.prometheus_k8s.v0.prometheus_remote_write was already up to date in version 0.5. Library charms.prometheus_k8s.v0.prometheus_scrape was already up to date in version 0.20. Library charms.traefik_k8s.v0.ingress_per_unit updated to version 0.10. ```

stderr

``` ```

[github-actions[bot]]

Libraries are not up to date with their remote counterparts. If this was not intentional, run charmcraft fetch-libs and commit the updated libs to your PR branch.

stdout

``` Library charms.alertmanager_k8s.v0.alertmanager_dispatch was already up to date in version 0.4. Library charms.grafana_k8s.v0.grafana_source was already up to date in version 0.10. Library charms.observability_libs.v0.kubernetes_service_patch was already up to date in version 0.6. Library charms.prometheus_k8s.v0.prometheus_remote_write was already up to date in version 0.5. Library charms.prometheus_k8s.v0.prometheus_scrape was already up to date in version 0.20. Library charms.traefik_k8s.v0.ingress_per_unit updated to version 0.10. ```

stderr

``` ```

[github-actions[bot]]

Libraries are not up to date with their remote counterparts. If this was not intentional, run charmcraft fetch-libs and commit the updated libs to your PR branch.

stdout

``` Library charms.alertmanager_k8s.v0.alertmanager_dispatch was already up to date in version 0.4. Library charms.grafana_k8s.v0.grafana_source was already up to date in version 0.10. Library charms.observability_libs.v0.kubernetes_service_patch was already up to date in version 0.6. Library charms.prometheus_k8s.v0.prometheus_remote_write was already up to date in version 0.5. Library charms.prometheus_k8s.v0.prometheus_scrape was already up to date in version 0.20. Library charms.traefik_k8s.v0.ingress_per_unit updated to version 0.10. ```

stderr

``` ```