hardware-exporter service wasn't installed when cos_agent relation is provided

[zxhdaze]

Similar to #281, I also found the config-change hook is called before hardware-exporter get installed.
I'm able to reproduce this error on a Redfish supported machine by:

juju deploy ubuntu
juju deploy hardware-observer
juju deploy grafana-agent
#To reproduce the error, relate grafana-agent to hardware-observer first
juju relate grafana-agent hardware-observer
juju relate hardware-observer ubuntu

The unit becomes blocked, asking to attach the storcli-deb resource and then requesting Redfish credentials. At this point, hardware-exporter is not installed. If I configure the charm with Redfish credentials at this stage, it crashes.

This error does not occur if the cos_agent relations are not provided. This is possibly related to #215.