Metrics can be missing due to cache expiration or connectivity issue
between the exporter and OpenStack APIs. Also see issue [1]
[1]: https://github.com/canonical/openstack-exporter-operator/issues/130
Change-Id: I081b7d6f704953ef3360ab6bbed20dcb2c59ec9f
Signed-off-by: Chi Wai Chan <chiwai.chan@canonical.com>