Grafana was the one swarm service that exports Prometheus metrics and had no
scrape target, so the swarm's metrics UI was invisible to the metrics it
displays.
It serves on a unix socket and claims no TCP port, which is deliberate and
stays that way; a prometheus target has to be a host:port, so nginx re-serves
the one endpoint a scraper needs on loopback. An exact-match location, not a
prefix: widening it would re-serve the whole UI without authorization.
The metrics section is now stated explicitly rather than inherited, because a
scrape target depends on it and a changed upstream default would take the
endpoint away while nginx kept answering.
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