report-basic: expose os-release fields as metrics

report-basic: expose os-release fields as metrics

Add io.systemd.Basic.OSRelease metric families that reports select fields
from os-release.

$ cat /etc/os-release
NAME="Fedora Linux"
VERSION="45 (Cloud Edition Prerelease)"
RELEASE_TYPE=development
ID=fedora
VERSION_ID=45
VERSION_CODENAME=""
PRETTY_NAME="Fedora Linux 45 (Cloud Edition Prerelease)"
ANSI_COLOR="0;38;2;60;110;180"
LOGO=fedora-logo-icon
CPE_NAME="cpe:/o:fedoraproject:fedora:45"
HOME_URL="https://fedoraproject.org/"
DOCUMENTATION_URL="https://docs.fedoraproject.org/en-US/fedora/rawhide/"
SUPPORT_URL="https://ask.fedoraproject.org/"
BUG_REPORT_URL="https://bugzilla.redhat.com/"
REDHAT_BUGZILLA_PRODUCT="Fedora"
REDHAT_BUGZILLA_PRODUCT_VERSION=rawhide
REDHAT_SUPPORT_PRODUCT="Fedora"
REDHAT_SUPPORT_PRODUCT_VERSION=rawhide
SUPPORT_END=2027-11-24
VARIANT="Cloud Edition"
VARIANT_ID=cloud

$ varlinkctl call --more ./build/systemd-report-basic io.systemd.Metrics.List {} | jq --seq -c
...
{"name":"io.systemd.Basic.OSRelease.NAME","value":"Fedora Linux 44 (Workstation Edition)"}
{"name":"io.systemd.Basic.OSRelease.ID","value":"fedora"}
{"name":"io.systemd.Basic.OSRelease.CPE_NAME","value":"cpe:/o:fedoraproject:fedora:44"}
{"name":"io.systemd.Basic.OSRelease.VARIANT_ID","value":"workstation"}
{"name":"io.systemd.Basic.OSRelease.VERSION_ID","value":"44"}
{"name":"io.systemd.Basic.OSRelease.SUPPORT_END","value":"2027-05-19"}

I picked the fields that contain useful information about the specific
version/image/variant/experiment/flavour of the system. Also, either
NAME or PRETTY_NAME is included. This one is intended for human readers
to be able to identify the OS version easily.