The builder pushes its build metrics (builds by kind and outcome, build
duration, queue depth and wait, reply publish failures) to its forwarder's
OTLP receiver, which relays them to the swarm collector.
It also registers the standard store client-certificate gauges
(bao.client_cert.*, cert=builder) on the same exporter. They are the
builder's liveness signal: exported every minute whether or not a build
runs. The cert-metrics table in docs/scheduler/observability.md gains the
builder row.
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