alloy: add prometheus.exporter.unix canary (Track A fleet rollout)

Embeds node_exporter inside Alloy alongside Loki shipping; pushes
metrics via remote_write to observe Prom with job=node_lxc_canary
to run side-by-side with the existing node_exporter scrape until
cutover. See homelab-docs/docs/audit/alloy-consolidation-2026-05-21.md.

roles/alloy/templates/config.alloy.j2

@@ -39,3 +39,49 @@ loki.write "default" {
     url = "{{ alloy_loki_url }}"
   }
 }
+
+// ============================================================
+// Prometheus — embedded node_exporter (replaces standalone).
+// Bare-metal Alloy on systemd; reads /proc, /sys, / natively
+// (no /host/* bind mounts; no pid:host needed).
+// ============================================================
+
+prometheus.exporter.unix "node" {
+  // Defaults: rootfs_path=/, procfs_path=/proc, sysfs_path=/sys
+}
+
+prometheus.scrape "node" {
+  targets         = prometheus.exporter.unix.node.targets
+  forward_to      = [prometheus.relabel.node.receiver]
+  scrape_interval = "15s"
+  job_name        = "{{ alloy_prom_job | default('node_lxc') }}"
+}
+
+// Alloy's exporter sets job=integrations/unix and the targets carry no
+// instance label by default. Rewrite to match the historical Prometheus
+// scrape labels so existing dashboards keep working.
+prometheus.relabel "node" {
+  forward_to = [prometheus.remote_write.observe.receiver]
+  rule {
+    target_label = "instance"
+    replacement  = "{{ alloy_prom_instance | default(ansible_default_ipv4.address + ':9100') }}"
+  }
+  rule {
+    target_label = "job"
+    replacement  = "{{ alloy_prom_job | default('node_lxc') }}"
+  }
+  rule {
+    target_label = "group"
+    replacement  = "{{ alloy_prom_group | default('lxc') }}"
+  }
+  rule {
+    target_label = "hostname"
+    replacement  = "{{ alloy_prom_hostname | default(alloy_host_label) }}"
+  }
+}
+
+prometheus.remote_write "observe" {
+  endpoint {
+    url = "{{ alloy_prom_remote_write | default('http://observe.lan.balders.ca:9090/api/v1/write') }}"
+  }
+}