Add to Promtail user_id, override unit if we have a _systemd_user_unit

[karandit]

Problem: we would like to see the logs also for user-level systemd services, until now we saw only accumulated from all services from a user which was inconvenient ([email protected])

Solution: Extend promtail to add also the user_id and in the case of user-level services use the value of the _systemd_user_unit instead of _systemd_unit which is usually init.scope