Upgrade from revision 27 or older

Revisions up until 27 included were intended for a custom build of snap golang-openstack-exporter, which needed to be provided as a resource as described in the tutorial.

Charm revisions newer than 27 switch support from the golang-openstack-exporter snap to the new charmed-openstack-exporter one, which delivers a newer snap release of the community openstack-exporter project and is intended to be used in conjunction with this charm.

Given the switch, we have decided to require operators to positively acknowledge the change by unsetting the custom resource during the upgrade procedure.

If you are upgrading from revision ≤ 27 to a revision > 27, the procedure will therefore look as follows:

# Start the upgrade
juju refresh openstack-exporter # the charm will move to a blocked state here

# Remove the old exporter
touch an_empty.snap # juju resources cannot be unset but only replaced with empty files
juju attach-resource openstack-exporter openstack-exporter=an_empty.snap
rm an_empty.snap

The charm will remove the previously installed golang-openstack-exporter, replace it with charmed-openstack-exporter (which will be automatically downloaded from the store), and move to the active state.