muster release v5.19.8

mcp-oauth’s metrics reach muster’s metrics endpoint, the token store’s included. The OAuth server’s instrumentation now records through muster’s own OpenTelemetry meter provider (mcp-oauth 1.4.0, instrumentation.Config.MeterProvider), and the Valkey and in-memory token stores share it. Until now mcp-oauth built a Prometheus collector of its own on the default registerer, which the :9464 endpoint – served from a registry of its own – never gathered: not one oauth_* series was ever exported, and the token store was not instrumented at all. Now oauth_http_requests_total, oauth_token_endpoint_failures_total{grant_type,error_code} and the other oauth_* series, storage_operation_total{operation,result} (result is success, error or timeout), the storage_operation_duration_milliseconds histogram and the storage_*_count size gauges are served next to the muster_* series. A Valkey outage shows as storage_operation_total{result="timeout"} and oauth_token_endpoint_failures_total{error_code="temporarily_unavailable"}.