swarm-otel: collect only the units the swarm's services declare

The journald receiver was configured with a directory and no filter, so
the swarm's log store received every unit on the host that runs the
collector. On a hive whose services live on a workstation that includes
the operator's desktop session, in a store every swarm operator can read.

The receiver has no system-only switch and its `matches` field is an
allowlist too, so what the swarm collects has to be stated rather than
excluded. Each service module names its own units: a service that is not
running contributes nothing, and one added later arrives declared.

An empty list is fail-open — the receiver renders no filter at all and
reads everything — so it is asserted against.

nix/host-modules/swarm-nats.nix

@@ -408,6 +408,13 @@ in
   };
 
   config = lib.mkIf cfg.enable {
+    # The responder as well as the server: a denial reaches the client as a
+    # timeout, so the server's own log is the only place it is an error.
+    services.hyperhive.swarm.otel.journaldUnits = [
+      "nats"
+      "swarm-nats-auth"
+    ];
+
     assertions = [
       {
         # Fail at EVAL, not at boot: a queue that comes up unable to