Alarm constructor statistic prop doesn't use Cfn extendedStatistic for percentile values

[mitchlloyd]

🐛 Bug Report

What is the problem?

Creating cloudwatch Alarms via the metric.createAlarm api works with percentage values.

Also creating a metric with a value like p95 correctly uses the extendedStatistic Cfn field on a Metric:

new cloudwatch.Alarm(this, 'MyAlarm', {
  // ...
  metric: new cloudwatch.Metric({
    statistic: 'p95', // Works!
    namespace: 'Namespace',
    metricName: 'Duration',
    dimensions: { 'Operation': 'MyAPI', 'Region': 'us-west-2', 'Service': 'MyService' },
  }),
});

However using the statistic field on alarm will not work. It only works with "simple" metrics because it only uses the statistic and never the extendedStatistic field.

new cloudwatch.Alarm(this, 'MyAlarm', {
  // ...
  statistic: 'p95', // Cfn error!
  metric: new cloudwatch.Metric({
    namespace: 'Namespace',
    metricName: 'Duration',
    dimensions: { 'Operation': 'MyAPI', 'Region': 'us-west-2', 'Service': 'MyService' },
  }),
});

This is quite a gotcha given that a "simple" metric works:

new cloudwatch.Alarm(this, 'GetCategorySuccessSampleCount', {
  // ...
  statistic: 'SampleCount', // Works!
  metric: new cloudwatch.Metric({
    namespace: 'Namespace',
    metricName: 'Success',
    dimensions: { 'Operation': 'MyAPI', 'Region': 'us-west-2', 'Service': 'MyService' },
  }),
});

Reproduction Steps

This causes a Cfn error when deploying:

new cloudwatch.Alarm(this, 'MyAlarm', {
  // ...
  statistic: 'p95', // Cfn error!
  metric: new cloudwatch.Metric({
    namespace: 'Namespace',
    metricName: 'Duration',
    dimensions: { 'Operation': 'MyAPI', 'Region': 'us-west-2', 'Service': 'MyService' },
  }),
});

Environment

• CDK CLI Version: 1.6.0
• Module Version: 1.6.0
• OS: OSX
• Language: TypeScript