An open circuit whose nextAttemptAt has passed stays open until
the next request arrives; that request is admitted as the half-open trial.
Consecutive failures, or failures in the rolling window when window is set.
OptionalopenedEpoch ms when the circuit last opened. Absent while closed.
OptionalnextEpoch ms when an open circuit admits its next trial (openedAt + resetTimeoutMs). Absent while closed.
Point-in-time view of one partition's circuit breaker, as returned by
client.circuits(). A plain copy: mutating it doesn't affect the breaker.