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>

tests/integration/test_docker.py

@@ -575,17 +575,26 @@ def test_swarm_topology_persisted(app):
                 s.add(h)
                 await s.flush()
                 # No containers in this sample — swarm must still persist.
+                from steward.core.plugin_manager import loaded_plugin_submodule
+                loaded_plugin_submodule("docker", "swarm_health")._last_recorded = None
+                since = datetime.now(timezone.utc)
                 await persist(s, h, [], swarm)
                 hid = h.id
+            # The swarm-wide figures alert rules watch (#5788): web is 2 of 3.
+            figures = dict((await s.execute(text(
+                "SELECT metric_name, value FROM plugin_metrics WHERE source_module = 'docker' "
+                "AND resource_name = 'swarm' AND recorded_at >= :t"), {"t": since})).all())
             svc = (await s.execute(text(
                 "SELECT mode, desired, running, image, placement_json, tasks_json "
                 "FROM docker_swarm_services WHERE host_id = :h"), {"h": hid})).first()
             node = (await s.execute(text(
                 "SELECT role, availability, status, leader "
                 "FROM docker_swarm_nodes WHERE host_id = :h"), {"h": hid})).first()
-            return svc, node
+            return svc, node, figures
 
-    svc, node = asyncio.run(_go())
+    svc, node, figures = asyncio.run(_go())
+    assert figures == {"swarm_services_short": 1.0, "swarm_nodes_down": 0.0,
+                       "swarm_managers_unreachable": 0.0}
     assert svc[0] == "replicated" and svc[1] == 3 and svc[2] == 2 and svc[3] == "nginx"
     assert "n1" in svc[4]  # placement JSON carries the node id
     (task,) = json.loads(svc[5])  # docker_010: the manager's task records survive