feat(aws-applicationautoscaling): enabling autoscaling for ElastiCache Redis cluster (#17919)

Following the recently released support for autoscaling in ElastiCache Redis cluster, I'd like to use CDK in order to manage the infrastructure. The only required change is to introduce a new enum value for 'elasticache' key ([cloudformation doc](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-applicationautoscaling-scalabletarget.html#cfn-applicationautoscaling-scalabletarget-servicenamespace)), however to improve dev experience I've introduced three new `PredefinedMetricType` following [cloudformation docs](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-applicationautoscaling-scalingpolicy-predefinedmetricspecification.html#cfn-applicationautoscaling-scalingpolicy-predefinedmetricspecification-predefinedmetrictype)

----

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

Author: astrohome

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

@@ -206,3 +206,20 @@ target.scaleToTrackMetric('PceTracking', {
   predefinedMetric: appscaling.PredefinedMetric.LAMBDA_PROVISIONED_CONCURRENCY_UTILIZATION,
 })
 ```
+
+### ElastiCache Redis shards scaling with target value
+
+```ts
+const shardsScalableTarget = new appscaling.ScalableTarget(this, 'ElastiCacheRedisShardsScalableTarget', {
+  serviceNamespace: appscaling.ServiceNamespace.ELASTICACHE,
+  scalableDimension: 'elasticache:replication-group:NodeGroups',
+  minCapacity: 2,
+  maxCapacity: 10,
+  resourceId: 'replication-group/main-cluster',
+});
+
+shardsScalableTarget.scaleToTrackMetric('ElastiCacheRedisShardsCPUUtilization', {
+  targetValue: 20,
+  predefinedMetric: appscaling.PredefinedMetric.ELASTICACHE_PRIMARY_ENGINE_CPU_UTILIZATION,
+});
+```

packages/@aws-cdk/aws-applicationautoscaling/lib/scalable-target.ts

@@ -281,4 +281,9 @@ export enum ServiceNamespace {
    * Kafka
    */
   KAFKA = 'kafka',
+
+  /**
+   * ElastiCache
+   */
+  ELASTICACHE = 'elasticache',
 }

packages/@aws-cdk/aws-applicationautoscaling/lib/target-tracking-scaling-policy.ts

@@ -243,4 +243,19 @@ export enum PredefinedMetric {
    * @see https://docs.aws.amazon.com/autoscaling/application/APIReference/API_PredefinedMetricSpecification.html
    */
   KAFKA_BROKER_STORAGE_UTILIZATION = 'KafkaBrokerStorageUtilization',
+  /**
+   * ELASTIC_CACHE_PRIMARY_ENGINE_CPU_UTILIZATION
+   * @see https://docs.aws.amazon.com/autoscaling/application/APIReference/API_PredefinedMetricSpecification.html
+   */
+  ELASTICACHE_PRIMARY_ENGINE_CPU_UTILIZATION = 'ElastiCachePrimaryEngineCPUUtilization',
+  /**
+   * ELASTIC_CACHE_REPLICA_ENGINE_CPU_UTILIZATION
+   * @see https://docs.aws.amazon.com/autoscaling/application/APIReference/API_PredefinedMetricSpecification.html
+   */
+  ELASTICACHE_REPLICA_ENGINE_CPU_UTILIZATION = 'ElastiCacheReplicaEngineCPUUtilization',
+  /**
+   * ELASTIC_CACHE_REPLICA_ENGINE_CPU_UTILIZATION
+   * @see https://docs.aws.amazon.com/autoscaling/application/APIReference/API_PredefinedMetricSpecification.html
+   */
+  ELASTICACHE_DATABASE_MEMORY_USAGE_COUNTED_FOR_EVICT_PERCENTAGE = 'ElastiCacheDatabaseMemoryUsageCountedForEvictPercentage',
 }

packages/@aws-cdk/aws-applicationautoscaling/test/scalable-target.test.ts

@@ -144,6 +144,7 @@ describe('scalable target', () => {
     expect(appscaling.ServiceNamespace.LAMBDA).toEqual('lambda');
     expect(appscaling.ServiceNamespace.RDS).toEqual('rds');
     expect(appscaling.ServiceNamespace.SAGEMAKER).toEqual('sagemaker');
+    expect(appscaling.ServiceNamespace.ELASTICACHE).toEqual('elasticache');
   });
 
   test('create scalable target with negative minCapacity throws error', () => {