SetupOTelSDK now adds service.instance.id (the pod hostname) to
OTEL_RESOURCE_ATTRIBUTES. The OTLP->Prometheus exporter maps it to the
`instance` label on both metrics and target_info, which (1) stops
multi-replica services from colliding on a single series and (2) gives
PromQL joins a unique (job, instance) key. Falls back to the service name
when the hostname is unavailable.
argoyle
