swarm-otel: stamp the swarm on every hive's pipeline

mara on the tracking issue: "we need the swarm label for upstream otel at
least (the out of swarm one)."

Stamped in the per-hive `resource` processor rather than on a separate
upstream-only pipeline, which would double the pipeline count to withhold one
constant label from the local store. It is redundant there — one metrics store
per swarm, so every series in it already belongs to this swarm — but a constant
label multiplies no series, and it means what leaves and what stays have the
same shape.

Upstream is where it stops being redundant: that is the one hop where several
swarms can land in one store, and samples that cannot name their swarm collide
there exactly as hives collided here before per-hive receivers existed.

`unknown` when unnamed rather than an absent label, copying the agent path so
a query never has to handle both "the label is missing" and "the label says
unknown".

nix/host-modules/swarm-otel.nix

@@ -70,6 +70,12 @@ let
   };
   caBundle = caTrust.bundlePathFor cfg.machine;
 
+  # `unknown` rather than omitting the label, copying `agent-modules/otel.nix`
+  # deliberately: a producer that cannot name its swarm should say so in the
+  # same vocabulary as every other producer, so a query never has to handle
+  # both "the label is absent" and "the label says unknown".
+  swarmDisplayName = if hyperhiveCfg.swarm.name == null then "unknown" else hyperhiveCfg.swarm.name;
+
   # One list, read by every pipeline: the per-hive pipelines fan out to
   # exactly the same destinations as the single pipeline they replace.
   # Written once because "which exporters" is a property of this tier, not
@@ -489,6 +495,24 @@ in
                       value = h;
                       action = "upsert";
                     }
+                    # Stamped here rather than on a separate upstream-only
+                    # pipeline, which would double the pipeline count to
+                    # withhold one constant label from the local store. It
+                    # is redundant there — one VictoriaMetrics per swarm, so
+                    # every series in it already belongs to this swarm — but
+                    # a constant label multiplies no series, and it means
+                    # what LEAVES and what STAYS have the same shape.
+                    #
+                    # Upstream is where it stops being redundant: that is the
+                    # one hop where several swarms can land in one store, and
+                    # samples that cannot name their swarm collide there
+                    # exactly as hives collided here before per-hive
+                    # receivers existed.
+                    {
+                      key = "swarm";
+                      value = swarmDisplayName;
+                      action = "upsert";
+                    }
                   ];
                 }
               ) hivePorts;