imds: expose instance metadata as an io.systemd.Metrics provider

When systemd-imds is invoked as a Varlink service (via the new
systemd-imds-metrics.socket), it now acts as an io.systemd.Metrics
provider for systemd-report. It connects to systemd-imdsd over the
existing io.systemd.InstanceMetadata interface to acquire the real
data and re-exposes the detected cloud vendor plus the well-known
hostname, region, zone and public IPv4/IPv6 fields as metrics in the
io.systemd.InstanceMetadata.* namespace.

The metrics logic lives entirely on the client side
(imds-tool-metrics.c); systemd-imdsd is unchanged. Each metric is
acquired on demand with a blocking call to the daemon, benefiting from
its local cache. Fields that are unset or unsupported by the vendor are
simply omitted.

The metrics socket is statically enabled into sockets.target.wants/.

units/meson.build

@@ -413,6 +413,14 @@ units = [
           'file' : 'systemd-imds-import.service.in',
           'conditions' : ['ENABLE_IMDS'],
         },
+        {
+          'file' : 'systemd-imds-metrics.socket',
+          'conditions' : ['ENABLE_IMDS'],
+        },
+        {
+          'file' : 'systemd-imds-metrics@.service.in',
+          'conditions' : ['ENABLE_IMDS'],
+        },
         {
           'file' : 'systemd-importd.service.in',
           'conditions' : ['ENABLE_IMPORTD'],

units/systemd-imds-metrics.socket

@@ -0,0 +1,22 @@
+#  SPDX-License-Identifier: LGPL-2.1-or-later
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Cloud Instance Metadata Metrics
+Documentation=man:systemd-imds(1)
+DefaultDependencies=no
+Before=sockets.target
+
+[Socket]
+ListenStream=/run/systemd/report/io.systemd.InstanceMetadata
+FileDescriptorName=varlink
+SocketMode=0666
+Accept=yes
+MaxConnectionsPerSource=16
+RemoveOnStop=yes

units/systemd-imds-metrics@.service.in

@@ -0,0 +1,25 @@
+#  SPDX-License-Identifier: LGPL-2.1-or-later
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Cloud Instance Metadata Metrics
+Documentation=man:systemd-imds(1)
+DefaultDependencies=no
+Conflicts=shutdown.target initrd-switch-root.target
+Before=shutdown.target initrd-switch-root.target
+After=systemd-imdsd.socket
+
+[Service]
+# Runs systemd-imds as an io.systemd.Metrics provider. It connects to
+# systemd-imdsd to acquire the actual data. Runs as root so the daemon's Polkit
+# check on io.systemd.InstanceMetadata.Get() passes.
+ExecStart={{LIBEXECDIR}}/systemd-imds
+LockPersonality=yes
+MemoryDenyWriteExecute=yes
+NoNewPrivileges=yes