The endpoint was off, so nothing reported on the swarm's own SSO. Enabling
it alone would have added no data — the scraper that reads it only landed
with the swarm-tier prometheus receiver.
Loopback only, like the main listener and for a stronger reason: this
endpoint authenticates nothing and reports request volumes and outcomes for
every login on the swarm.
metricsPort is an option rather than a literal because every swarm container
shares the host netns, so two services picking the same port do not conflict
at build time — one loses at runtime with nothing in any log. 9959 is
upstream's default and is unclaimed across nix/.
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