A collector's `refused` / `failed` / queue-depth counters are the only
signal that says telemetry is being dropped, and nothing read them at
any tier — so a collector losing records looked exactly like a quiet
system.
The hive tier could not be scraped without first naming its port. 8888
is the collector's built-in default and appeared in no config, which is
also why nothing comparing configured ports could see it clash with a
co-located collector — swarm-otel.nix sidesteps 8888 by hand for that
reason, and says so. Declaring the port and binding it explicitly makes
the value comparable; wiring the scrape is then one entry per tier.
Extending the port-collision assertion to cover it is deliberately left
out: that belongs with the other port work, and coupling a collision
fix to a scraping fix makes both harder to review.
Gate: 101 module properties hold, was 95. The six cases pin the rendered
scrape job rather than the option; the metrics pipeline naming the
prometheus receiver, a path never emitted on any hive before this since
the hive tier's scrapeTargets was empty everywhere; the `readers`
spelling, with a control so a missing telemetry block cannot pass the
port check vacuously; the swarm tier's own entry; the two tiers not
claiming the same port; and the absence arm, a hive with no collector
declaring no target.
scrapeTargets' description said "Empty by default, and that is the
shipped case". This makes that false, so the paragraph moves with it.
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