fix: run alloy in the host cgroup namespace (cgroup: host)

The cadvisor exporter reads /sys/fs/cgroup for per-container stats, but with the
default private cgroup namespace it only saw the container's own cgroup — so it
emitted machine-root container_* series (one per node, no name label) and no
per-container metrics. cgroup: host exposes the host cgroup tree so cadvisor
enumerates every container (mapped to names via the docker socket).

compositions/apex/docker-compose.yml

@@ -192,6 +192,7 @@ services:
     image: grafana/alloy:v1.17.1
     container_name: apex-alloy
     hostname: "${APEX_NODE_HOST}"   # UTS hostname => node_exporter's node_uname_info `nodename` (else the container id)
+    cgroup: host                    # host cgroup namespace: the cadvisor exporter needs it to read per-container cgroups (else machine-root only)
     profiles: [alloy]
     user: "0:0"  # root: required for host metrics, journal, and docker.sock
     volumes: