runtime: Fix opentelemetry gracefull shutdown

When running OPA with the distributed tracing option enabled,
the OpenTelemetry trace exporter is not gracefully shut down
when the server is stopped.

This PR fixes that issues by moving the trace exporter shutdown
in the gracefulShutdonw function.

Fixes: open-policy-agent#6651
Signed-off-by: Nicolas Chotard <nicolas.chotard@backmarket.com>

runtime/runtime.go

@@ -534,13 +534,6 @@ func (rt *Runtime) Serve(ctx context.Context) error {
 			rt.logger.WithFields(map[string]interface{}{"err": err}).Error("Failed to start OpenTelemetry trace exporter.")
 			return err
 		}
-
-		defer func() {
-			err := rt.traceExporter.Shutdown(ctx)
-			if err != nil {
-				rt.logger.WithFields(map[string]interface{}{"err": err}).Error("Failed to shutdown OpenTelemetry trace exporter gracefully.")
-			}
-		}()
 	}
 
 	rt.server = server.New().
@@ -857,6 +850,13 @@ func (rt *Runtime) gracefulServerShutdown(s *server.Server) error {
 		return err
 	}
 	rt.logger.Info("Server shutdown.")
+
+	if rt.traceExporter != nil {
+		err = rt.traceExporter.Shutdown(ctx)
+		if err != nil {
+			rt.logger.WithFields(map[string]interface{}{"err": err}).Error("Failed to shutdown OpenTelemetry trace exporter gracefully.")
+		}
+	}
 	return nil
 }