dashboards_backup script does not work with grafana version >= 8

[Mustafa Kemal Gilor]

dashboards_backup script is using a deprecated API to retrieve the dashboard JSON information from Grafana. The used API[1] is deprecated in version 5.4 in favor of using dashboard UID's[2] and removed in version 8.0.

To reproduce the issue, deploy the Grafana charm with APT[3] as follows:

1) Install Grafana charm with APT install method:
juju deploy grafana grafanasnap --series bionic --config install_method=apt

2) Login to Grafana web UI & create a dashboard

3) Schedule automatic dashboard backup job:
juju config grafanasnap dashboards_backup_schedule='*/5 * * * *'

4) Retrieve the backup command from the scheduled job:
sudo cat /etc/cron.d/juju-dashboards-backup

5) Run the backup command by hand:
/usr/local/bin/dashboards_backup -d /srv/backups -p 3000 -s http <redacted>

6) Check dashboard backup files in the backup folder:
/srv/backups/Main_Org_# cat grafana-metrics.json.20220804_132501
{"message": "Not found"}

The backup files are supposed to have the dashboard contents. Instead, they have `{"message": "Not found"}` since the API is not present in the deployed Grafana version.

Remedy:
The backup script should use the UID API for Grafana version >= 8.

[1]: https://grafana.com/docs/grafana/v5.4/http_api/dashboard/#get-dashboard-by-slug
[2]: https://grafana.com/docs/grafana/v5.4/http_api/dashboard/#get-dashboard-by-uid
[3]: APT package's grafana version is 9.0.6, whereas snap package's grafana version is 7.4.1