mgr/prometheus: fix metrics service not coming up
https://github.com/ceph/ceph/pull/61468 unintentionally broke the http
metric service while it removed the code that starts the metrics. adding
them back up.
adding a basic test to catch these issues
Regression from https://github.com/ceph/ceph/pull/61468/commits/
64f590cc8f03c9aab909c3ea5b9f53271ba3c15b#diff-
031e09c4297d84a407cf55f8981d38764efc3c37e9827e12e638521f69284e1f
Fixes: https://tracker.ceph.com/issues/72012
Signed-off-by: Nizamudeen A <nia@redhat.com>