A collector serves its own metrics on localhost:8888 unless told
otherwise, and co-located tiers share a network namespace, so the second
one to start dies with 'bind: address already in use'.
The port appears in neither config - it is a default inside the binary -
so comparing the ports the configs name reports them distinct. A
behavioural probe found it by being unable to start the chain.
metrics.address is the spelling that looks right and is rejected by this
version ('migration.MetricsConfigV030' has invalid keys: address);
readers is the schema it accepts.
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