feat(diagnostics): retain completed event-loop observation windows (#…

feat(diagnostics): retain completed event-loop observation windows (#138249)

Record completed health-monitor windows through existing diagnostics so
Prometheus and OpenTelemetry retain intervals consumed by earlier readers.
Preserve readiness and lifecycle behavior and exclude reader trace identity.

Cover cumulative retention, disabled consumers, queue and series limits,
real SDK context handling, and managed-install Gateway authentication and
graceful shutdown.