stackdriver: fix typescript error

public/app/plugins/datasource/stackdriver/specs/query_ctrl.test.ts

@@ -417,6 +417,7 @@ function createTarget(existingFilters?: string[]) {
       id: '',
       name: '',
     },
+    unit: '',
     metricType: 'ametric',
     service: '',
     refId: 'A',