feat: expose graceful drain state

internal/observability/catalog.go

@@ -29,6 +29,7 @@ func catalog() map[string]descriptor {
 		"orbit_runtime_batch_last_success_timestamp_seconds": {gauge, "Unix time of last successful background batch, zero before first success", map[string][]string{"component": components}},
 		"orbit_process_start_time_seconds":                   {gauge, "Unix time when the Orbit metrics registry was started", nil},
 		"orbit_process_uptime_seconds":                       {gauge, "Monotonic elapsed time since metrics registry startup", nil},
+		"orbit_process_draining":                             {gauge, "Whether this process is draining for graceful shutdown", nil},
 		"orbit_go_goroutines":                                {gauge, "Current Go goroutine count", nil},
 		"orbit_build_info":                                   {gauge, "Orbit build and protocol information", map[string][]string{"go_version": {build.GoVersion}, "version": {build.Version}, "revision": {build.Revision}}},
 	}