feat(swarm): services short, nodes down and managers lost are alertable (#5788)

On 2026-10-14 nine swarm services sat at 0/1 and nothing fired. From each
manager's swarm report the docker plugin now records three figures on
resource "swarm": swarm_services_short, swarm_nodes_down and
swarm_managers_unreachable. Drained nodes are left out of the last two,
since a drain is deliberate. The figures are recorded at most once every
30 s whichever manager reported, and are offered to alert rules.

Agent 1.19.0 sends each manager's ManagerStatus.Reachability. Older agents
fall back to the node's status.

A maintenance job on a swarm node also keeps a window over docker/swarm,
so its drain and reboot hold those alerts until the job ends.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/docker/__init__.py

@@ -83,6 +83,10 @@ def get_alert_metrics() -> dict[str, list[str]]:
     """What ingest.py records per container (resource "<host>/<container>"):
     restart_count is cumulative (alert on crash-looping); is_healthy is 1.0 /
     0.0 from the container's HEALTHCHECK, only for containers that define one.
-    Per host (resource "<host>"), what Docker keeps on its disk (#5304)."""
+    Per host (resource "<host>"), what Docker keeps on its disk (#5304).
+    Swarm-wide (resource "swarm"), from any manager's report: services short
+    of their replicas, nodes down and managers unreachable, drained nodes
+    aside (#5788, swarm_health.py)."""
     from .disk_view import DISK_METRICS
-    return {"docker": ["cpu_pct", "mem_pct", "restart_count", "is_healthy", *DISK_METRICS]}
+    from .swarm_health import SWARM_METRICS
+    return {"docker": ["cpu_pct", "mem_pct", "restart_count", "is_healthy", *DISK_METRICS, *SWARM_METRICS]}