feat(wafv2): better handle contextually required region prop (#535)

1. Throw an error at build time if we're missing the prop when it's
needed.
2. Automatically get the region when needed when using `monitorScope`.

---

_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

@@ -1,4 +1,4 @@
-import { IAspect } from "aws-cdk-lib";
+import { IAspect, Stack } from "aws-cdk-lib";
 import * as apigw from "aws-cdk-lib/aws-apigateway";
 import * as apigwv2 from "aws-cdk-lib/aws-apigatewayv2";
 import * as appsync from "aws-cdk-lib/aws-appsync";
@@ -430,8 +430,14 @@ export class MonitoringAspect implements IAspect {
       this.props.webApplicationFirewallAclV2,
     );
     if (isEnabled && node instanceof wafv2.CfnWebACL) {
+      const regionProps: Record<string, string> = {};
+      if (node.scope === "REGIONAL") {
+        regionProps.region = Stack.of(node).region;
+      }
+
       this.monitoringFacade.monitorWebApplicationFirewallAclV2({
         acl: node,
+        ...regionProps,
         ...props,
       });
     }

lib/monitoring/aws-wafv2/WafV2MetricFactory.ts

@@ -28,6 +28,10 @@ export class WafV2MetricFactory extends BaseMetricFactory<WafV2MetricFactoryProp
   constructor(metricFactory: MetricFactory, props: WafV2MetricFactoryProps) {
     super(metricFactory, props);
 
+    if (props.acl.scope === "REGIONAL" && !props.region) {
+      throw new Error(`region is required if CfnWebACL has "REGIONAL" scope`);
+    }
+
     this.dimensions = {
       Rule: AllRulesDimensionValue,
       WebACL: props.acl.name,

test/facade/__snapshots__/MonitoringAspect.test.ts.snap

@@ -27,6 +27,26 @@ test("snapshot test: no alarms", () => {
   expect(Template.fromStack(stack)).toMatchSnapshot();
 });
 
+test("with REGIONAL ACL but no region prop, throws error", () => {
+  const stack = new Stack();
+  const acl = new CfnWebACL(stack, "DummyAcl", {
+    name: "DummyAclName",
+    defaultAction: { allow: {} },
+    scope: "REGIONAL",
+    visibilityConfig: {
+      sampledRequestsEnabled: true,
+      cloudWatchMetricsEnabled: true,
+      metricName: "DummyMetricName",
+    },
+  });
+
+  const scope = new TestMonitoringScope(stack, "Scope");
+
+  expect(() => new WafV2Monitoring(scope, { acl })).toThrow(
+    `region is required if CfnWebACL has "REGIONAL" scope`,
+  );
+});
+
 test("snapshot test: all alarms", () => {
   const stack = new Stack();
   const acl = new CfnWebACL(stack, "DummyAcl", {