otel: agents reach their hive's collector by a swarm-wide name

Agents are handed `http://hive-collector.<swarm domain>:<port>` as their
otel endpoint instead of `http://<bridge IP>:<port>`. Every hive running
its collector contributes the name to `gateway.localNames`, so its own
dnsmasq answers it with its bridge IP, the way the queue's name already
works. The rendered meta flake then carries no hive-specific address,
which a swarm-level render needs (#4857, operator ruling 100744: "b").

hive-c0re's own exporter keeps the bridge address: on the host the name
may resolve to loopback through `gateway.localHostsEntry`, where the
receiver does not listen.

The name sits under the swarm domain and starts with `hive`, a fragment
no hive name may contain, so no hive's default domain can take it.

hive-c0re/src/stats/otel_metrics.rs

@@ -2,7 +2,8 @@
 //! sampled in [`super::container_stats`] feeds: it rides those gauges out to the
 //! hive's own collector — the same first hop the agents use, named by the
 //! standard `OTEL_EXPORTER_OTLP_ENDPOINT` (see `nix/host-modules/hive-c0re`,
-//! which sets it from the same binding it hands agents). No toggle of
+//! which sets it to the collector's bridge address where agents get its
+//! swarm-wide name). No toggle of
 //! its own, and no credential: a hive's collector takes unauthenticated OTLP
 //! on the bridge, and the only hop that presents anything is the swarm tier's,
 //! which is the one that leaves the swarm.