feat: expose graceful drain state

internal/observability/runtime.go

@@ -32,12 +32,26 @@ func (r *Registry) RecordRuntimeConfig(config RuntimeConfig) {
 	r.SetGauge("orbit_shutdown_timeout_seconds", config.ShutdownTimeout.Seconds(), nil)
 }
 
+// RecordDraining records whether this process has entered graceful shutdown.
+// The value is a fixed, node-local gauge: 0 means serving and 1 means draining.
+func (r *Registry) RecordDraining(draining bool) {
+	if r == nil {
+		return
+	}
+	value := 0.0
+	if draining {
+		value = 1
+	}
+	r.SetGauge("orbit_process_draining", value, nil)
+}
+
 // initializeProcess is called once by the registry constructor.
 func (r *Registry) initializeProcess() {
 	if r == nil {
 		return
 	}
 	r.SetGauge("orbit_process_start_time_seconds", float64(r.started.Unix()), nil)
+	r.RecordDraining(false)
 	build := buildinfo.Current()
 	r.SetGauge("orbit_build_info", 1, Labels{"go_version": build.GoVersion, "version": build.Version, "revision": build.Revision})
 	r.RecordProcess()