[System] syslog/auth journald pipeline drops systemd.* fields #21773

The journald pipeline for the system.syslog and system.auth data streams removes journald, systemd, syslog, process.thread and
message_id (added in #11618). This drops systemd.unit, which is the reliable way to tie a journald line to a service: process.name is
the syslog identifier, is empty for kernel lines and does not match the unit name.

Please keep systemd.* (at least systemd.unit), or make the removal optional.

Version: system 3.0.0