fix(cloudwatch): can use percentile override in Alarm (#4253)

Correctly render percentile statistic to the `ExtendedStatistic` field, instead
of the `Statistic` field.

Fixes #3845.

packages/@aws-cdk/aws-cloudwatch/lib/alarm.ts

@@ -4,7 +4,7 @@ import { CfnAlarm } from './cloudwatch.generated';
 import { HorizontalAnnotation } from './graph';
 import { CreateAlarmOptions } from './metric';
 import { IMetric } from './metric-types';
-import { normalizeStatistic } from './util.statistic';
+import { parseStatistic } from './util.statistic';
 
 export interface IAlarm extends IResource {
   /**
@@ -147,7 +147,8 @@ export class Alarm extends Resource implements IAlarm {
       ...dropUndef({
         // Alarm overrides
         period: props.period && props.period.toSeconds(),
-        statistic: props.statistic && normalizeStatistic(props.statistic),
+        statistic: renderIfSimpleStatistic(props.statistic),
+        extendedStatistic: renderIfExtendedStatistic(props.statistic),
       })
     });
 
@@ -248,3 +249,25 @@ function dropUndef<T extends object>(x: T): T {
   }
   return ret;
 }
+
+function renderIfSimpleStatistic(statistic?: string): string | undefined {
+  if (statistic === undefined) { return undefined; }
+
+  const parsed = parseStatistic(statistic);
+  if (parsed.type === 'simple') {
+    return parsed.statistic;
+  }
+  return undefined;
+}
+
+function renderIfExtendedStatistic(statistic?: string): string | undefined {
+  if (statistic === undefined) { return undefined; }
+
+  const parsed = parseStatistic(statistic);
+  if (parsed.type === 'percentile') {
+    // Already percentile. Avoid parsing because we might get into
+    // floating point rounding issues, return as-is but lowercase the p.
+    return statistic.toLowerCase();
+  }
+  return undefined;
+}

packages/@aws-cdk/aws-cloudwatch/test/test.alarm.ts

@@ -86,6 +86,32 @@ export = {
     test.done();
   },
 
+  'can use percentile in Alarm'(test: Test) {
+    // GIVEN
+    const stack = new Stack();
+
+    // WHEN
+    new Alarm(stack, 'Alarm', {
+      metric: testMetric,
+      statistic: 'P99',
+      threshold: 1000,
+      evaluationPeriods: 3,
+    });
+
+    // THEN
+    expect(stack).to(haveResource('AWS::CloudWatch::Alarm', {
+      ComparisonOperator: "GreaterThanOrEqualToThreshold",
+      EvaluationPeriods: 3,
+      MetricName: "Metric",
+      Namespace: "CDK/Test",
+      Period: 300,
+      ExtendedStatistic: 'p99',
+      Threshold: 1000,
+    }));
+
+    test.done();
+  },
+
   'can set DatapointsToAlarm'(test: Test) {
     // GIVEN
     const stack = new Stack();