fix(apigateway): use deployed stage's name when using MonitoringAspect (#409)

Fixes #381

---

_By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license_

Author: Eugene Cheung

lib/facade/MonitoringAspect.ts

@@ -106,6 +106,7 @@ export class MonitoringAspect implements IAspect {
     if (isEnabled && node instanceof apigw.RestApi) {
       this.monitoringFacade.monitorApiGateway({
         api: node,
+        apiStage: node.deploymentStage.stageName,
         ...props,
       });
     }

test/facade/MonitoringAspect.test.ts

@@ -89,6 +89,18 @@ describe("MonitoringAspect", () => {
     const api = new apigw.RestApi(stack, "DummyRestApi");
     api.root.addMethod("ANY");
 
+    const deployment = new apigw.Deployment(stack, "DummyRestApiDeployment", {
+      api,
+    });
+    new apigw.Stage(stack, "DummyRestApiDevStage", {
+      deployment,
+      stageName: "DevStage",
+    });
+    api.deploymentStage = new apigw.Stage(stack, "DummyRestApiProdStage", {
+      deployment,
+      stageName: "ProdStage",
+    });
+
     // WHEN
     facade.monitorScope(stack, defaultAspectProps);