fix(aws-cloudwatch): unable to use generic extended statistics for cloudwatch alarms

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

@@ -392,7 +392,10 @@ function renderIfExtendedStatistic(statistic?: string): string | undefined {
     // Already percentile. Avoid parsing because we might get into
     // floating point rounding issues, return as-is but lowercase the p.
     return statistic.toLowerCase();
+  } else if (parsed.type === 'generic') {
+    return statistic;
   }
+
   return undefined;
 }
 

packages/@aws-cdk/aws-cloudwatch/lib/private/statistic.ts

@@ -53,13 +53,13 @@ export function parseStatistic(stat: string): SimpleStatistic | PercentileStatis
 
   return {
     type: 'generic',
-    statistic: lowerStat,
+    statistic: stat,
   };
 }
 
 export function normalizeStatistic(stat: string): string {
   const parsed = parseStatistic(stat);
-  if (parsed.type === 'simple') {
+  if (parsed.type === 'simple' || parsed.type === 'generic') {
     return parsed.statistic;
   } else {
     // Already percentile. Avoid parsing because we might get into

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

@@ -257,6 +257,26 @@ describe('Alarm', () => {
 
 
   });
+
+  test('can use a generic string for extended statistic to make alarm', () => {
+    // GIVEN
+    const stack = new Stack();
+
+    // WHEN
+    testMetric.createAlarm(stack, 'Alarm', {
+      threshold: 1000,
+      evaluationPeriods: 2,
+      statistic: 'tm99.9999999999',
+    });
+
+    // THEN
+    expect(stack).toHaveResource('AWS::CloudWatch::Alarm', {
+      Statistic: ABSENT,
+      ExtendedStatistic: 'tm99.9999999999',
+    });
+
+  });
+
 });
 
 class TestAlarmAction implements IAlarmAction {

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

@@ -232,12 +232,13 @@ describe('Metrics', () => {
   });
 
   test('metric accepts a variety of statistics', () => {
-    new Metric({
+    const customStat = 'myCustomStatistic';
+    const metric = new Metric({
       namespace: 'Test',
       metricName: 'Metric',
-      statistic: 'myCustomStatistic',
+      statistic: customStat,
     });
 
-
+    expect(metric.statistic).toEqual(customStat);
   });
 });