chore(elasticloadbalancingv2): specific 5XX CloudWatch metrics for ALB (

aws#30659)

### Reason for this change
ALB supports metrics for specific load balancer generated 5XX level error metrics.
https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-cloudwatch-metrics.html

However, they are currently not included in `HttpCodeElb`, so the `metrics.httpCodeElb` method is not available and `metrics.custom` must be used.
```ts
let alb: IApplicationLoadBalancer;
alb.metrics.httpCodeElb(HttpCodeElb.ELB_5XX_COUNT); // we can do this
alb.metrics.httpCodeElb(HttpCodeElb.ELB_500_COUNT); // currently we cannot do this
alb.metrics.custom("HTTPCode_ELB_500_Count");
```

Adding the metric names to `HttpCodeElb` makes it easier to understand as it can be configured in the same way as `HTTPCode_ELB_5XX_Count`.

### Description of changes
Add metrics names to `HttpCodeElb`.



### Description of how you validated changes



### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk-lib/aws-elasticloadbalancingv2/lib/alb/application-load-balancer.ts

@@ -753,6 +753,26 @@ export enum HttpCodeElb {
    * The number of HTTP 5XX server error codes that originate from the load balancer.
    */
   ELB_5XX_COUNT = 'HTTPCode_ELB_5XX_Count',
+
+  /**
+   * The number of HTTP 500 server error codes that originate from the load balancer.
+   */
+  ELB_500_COUNT = 'HTTPCode_ELB_500_Count',
+
+  /**
+   * The number of HTTP 502 server error codes that originate from the load balancer.
+   */
+  ELB_502_COUNT = 'HTTPCode_ELB_502_Count',
+
+  /**
+   * The number of HTTP 503 server error codes that originate from the load balancer.
+   */
+  ELB_503_COUNT = 'HTTPCode_ELB_503_Count',
+
+  /**
+   * The number of HTTP 504 server error codes that originate from the load balancer.
+   */
+  ELB_504_COUNT = 'HTTPCode_ELB_504_Count',
 }
 
 /**

packages/aws-cdk-lib/aws-elasticloadbalancingv2/test/alb/load-balancer.test.ts

@@ -684,6 +684,29 @@ describe('tests', () => {
     }
   });
 
+  test.each([
+    elbv2.HttpCodeElb.ELB_500_COUNT,
+    elbv2.HttpCodeElb.ELB_502_COUNT,
+    elbv2.HttpCodeElb.ELB_503_COUNT,
+    elbv2.HttpCodeElb.ELB_504_COUNT,
+  ])('use specific load balancer generated 5XX metrics', (metricName) => {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const vpc = new ec2.Vpc(stack, 'Stack');
+    const lb = new elbv2.ApplicationLoadBalancer(stack, 'LB', { vpc });
+
+    // WHEN
+    const metric = lb.metrics.httpCodeElb(metricName);
+
+    // THEN
+    expect(metric.namespace).toEqual('AWS/ApplicationELB');
+    expect(metric.statistic).toEqual('Sum');
+    expect(metric.metricName).toEqual(metricName);
+    expect(stack.resolve(metric.dimensions)).toEqual({
+      LoadBalancer: { 'Fn::GetAtt': ['LB8A12904C', 'LoadBalancerFullName'] },
+    });
+  });
+
   test('loadBalancerName', () => {
     // GIVEN
     const stack = new cdk.Stack();