cephadm: prometheus: The generatorURL in alerts is only using hostname

cephadm: prometheus: The generatorURL in alerts is only using hostname

Prometheus is currently using only the hostname in the 'generatorURL' of an alert which causes issues when clicking on the URL in the Ceph Dashboard or somewhere else, because in most cases the hostname of the node that is running the Prometheus container is not resolvable.

To fix that the command line argument '--web.external-url' must be appended in the systemd unit file of the Prometheus container, e.g. '--web.external-url http://foo.bar:9095' whereas a FQDN hostname is used.

Fixes: https://tracker.ceph.com/issues/55595
Signed-off-by: Volker Theile <vtheile@suse.com>
(cherry picked from commit 4281dc1bbc466dd061781a984b34bb0eafaf482f)