Promote systemd_unit from a packed field to a label in Loki

In the promtail config (hosted-loki step in ci-registry), we're currently packing the systemd_unit as a field. It would be very useful for devs hunting for a problem globally to be able to efficiently search for it, and the set of systemd units should be fixed and thus a reasonable increase in cardinality.

https://github.com/openshift/release/blob/master/ci-operator/step-registry/ipi/install/hosted-loki/ipi-install-hosted-loki-commands.sh

Stop packing this field, and instead promote it to in labelallow. Docs for testing on your own cluster are available here: https://docs.google.com/document/d/1jP6NVIiaPGBXukUX_OZLlB3WrnFDsPNN1cDMVB2ciBs/edit?usp=sharing, but in this case it may be simpler to just let CI do it via a pj-rehearse on a single e2e job, as this has a very high likelihood of working first try.