feat(#3265): feed the store from the collector, and derive the pair

Review feedback: the metrics pair had nothing writing into it, and it sat
outside the switch that turns on every other swarm-wide service.

The collector now exports to VictoriaMetrics as well as upstream — a
fan-out, not a choice: a local store is for looking at this swarm, an
upstream is for whoever aggregates across swarms.

That makes a local store a complete destination on its own, so
`otel.endpoint` is no longer required when it runs here; a hive with
neither is still refused. The assertion only ever relaxes, so every
config that evaluated before still does.

`enableRequiredServices` now derives both halves, alongside matrix,
authelia and nats. They derive together because a store with no UI is
unreadable and a UI with no store is empty.

nix/host-modules/swarm-required-services.nix

@@ -58,5 +58,17 @@ in
     # mode was minting the queue's callout nkeys and then never starting
     # the queue they authenticate against.
     nats.enable = lib.mkDefault swarmCfg.enableRequiredServices;
+
+    # The metrics pair. Once per swarm and optional, so they meet the rule
+    # in the option's description the same way the three above do — a hive
+    # that is not the service host is a *client* of this Grafana, not a
+    # second one.
+    #
+    # They derive together on purpose: a store with no UI is unreadable and
+    # a UI with no store is empty, so there is no sensible deployment that
+    # takes one and not the other from this switch. An operator who wants
+    # exactly one still sets it directly, which `mkDefault` allows.
+    victoriametrics.enable = lib.mkDefault swarmCfg.enableRequiredServices;
+    grafana.enable = lib.mkDefault swarmCfg.enableRequiredServices;
   };
 }