feat(elbv2): add metrics to INetworkLoadBalancer and IApplicationLoad…

…Balancer

By moving the metrics methods to the `INetworkLoadBalancer` and
`IApplicationLoadBalancer` interfaces it allows to create these metrics also for
LBs that are imported via the `fromXXX` methods.

To create the metrics for LBs requires only the full name of the LB. This
attribute is available at the constructs returned by the `fromXXX` methods.

To solve this problem I did:

- Introduce a new interface for each LB type: `INetworkLoadBalancerMetrics`,
`IApplicationLoadBalancerMetrics`
- Create a concrete implementation for the new interfaces (1 for each):
`NetworkLoadBalancerMetrics` and `ApplicationLoadBalancerMetrics`
- Make each concrete implementation of each Load Balancer to also provide a
`metrics` field. The concrete implementations of the load balancers are:
`ImportedApplicationLoadBalancer`, `LookedUpApplicationLoadBalancer`,
`ApplicationLoadBalancer` (and the same for the NLB classes).

I chose to create a new interface because code can be reused across the 3
concrete implementations of each Load Balancer. I deprecated the `metricXXX()`
methods of each load balancer because I think it is cleaner to access metrics
through the new `metrics` attribute/interface.

packages/@aws-cdk/aws-elasticloadbalancingv2/README.md

@@ -604,3 +604,14 @@ const listener = elbv2.NetworkListener.fromLookup(this, 'ALBListener', {
   listenerPort: 12345,
 });
 ```
+
+## Metrics
+
+You may create metrics for each Load Balancer through the `metrics` attribute:
+
+```ts
+declare const alb: elbv2.IApplicationLoadBalancer;
+
+const albMetrics: elbv2.IApplicationLoadBalancerMetrics = alb.metrics;
+const metricConnectionCount: cloudwatch.Metric = albMetrics.activeConnectionCount();
+```