Preserve alert state when telemetry disappears

For Grafana alerts, distinguish a complete below-threshold observation from unknown telemetry. A filtering Prometheus predicate returns no series for both; configuring No Data as Normal can then resolve an outage when measurement disappears. Emit an explicit zero for known recovery and no data for uncertainty, with KeepLast handling. Keep a stable incident identity: partial disappearance of provider or family series can otherwise trigger MissingSeries resolution even when No Data uses KeepLast. This design prevents false recovery; it does not establish complete fleet coverage or notification delivery. Qualify those boundaries separately.

Applies when: correlated impact alerts must stay open through telemetry loss. Keywords: Grafana Prometheus KeepLast MissingSeries telemetry unknown recovery incident