feat(cloudwatch): allow arbitrary statistics in Metric and Alarm (aws…

…#15387)

This change maintains the existing logic which parses statistic strings provided by the user for Simple statistics or Percentile statistics, but it also allows for any string to be passed.

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

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

@@ -9,10 +9,16 @@ export interface PercentileStatistic {
   type: 'percentile';
   percentile: number;
 }
+
+export interface GenericStatistic {
+  type: 'generic';
+  statistic: string;
+}
+
 /**
  * Parse a statistic, returning the type of metric that was used (simple or percentile)
  */
-export function parseStatistic(stat: string): SimpleStatistic | PercentileStatistic {
+export function parseStatistic(stat: string): SimpleStatistic | PercentileStatistic | GenericStatistic {
   const lowerStat = stat.toLowerCase();
 
   // Simple statistics
@@ -45,7 +51,10 @@ export function parseStatistic(stat: string): SimpleStatistic | PercentileStatis
     };
   }
 
-  throw new Error(`Not a valid statistic: '${stat}', must be one of Average | Minimum | Maximum | SampleCount | Sum | pNN.NN`);
+  return {
+    type: 'generic',
+    statistic: lowerStat,
+  };
 }
 
 export function normalizeStatistic(stat: string): string {

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

@@ -231,4 +231,14 @@ export = {
 
     test.done();
   },
+
+  'metric accepts a variety of statistics'(test: Test) {
+    new Metric({
+      namespace: 'Test',
+      metricName: 'Metric',
+      statistic: 'myCustomStatistic',
+    });
+
+    test.done();
+  },
 };