Health check

CircuitBreakers reports the state ("open" or "closed") of each upstream registry's artifact-fetch circuit breaker, keyed by the host fetched from or, where the fetch URL has none to read, by an opaque placeholder standing in for it. It is omitted when no breaker has been created yet. An open breaker fails every artifact fetch it covers without contacting the upstream, but says nothing about this proxy's own health, so it does not change Status.

UpstreamHolds lists upstream hosts that rate limited the proxy and the RFC 3339 time until which it sends them nothing. A held host fails cache misses without contacting it and does not change Status.