B1: add AlarmProviderSwitchCount to GatewayMetricsSnapshot so the switch total is
readable without scraping the OTEL counter.
B2: replace the free-text reason tag on mxgateway.alarms.provider_switches with a
bounded AlarmProviderSwitchReason enum (failover/failback/unknown); the human-readable
reason stays in the structured log.
Joseph Doherty