fix(cloudwatch): unrecognized statistic warning when using percentileRank statistic in Stats helper (#29498)

### Issue # (if applicable)

Closes #29465.

### Reason for this change

There shouldn't be a warning when `Stats.percentileRank`

### Description of changes

Add a new parser for percentileRank statistic

### Description of how you validated changes

unit test

### Checklist
- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

Author: xazhao

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

@@ -34,6 +34,10 @@ export interface PercentileStatistic extends SingleStatistic {
   statName: 'percentile';
 }
 
+export interface PercentileRankStatistic extends PairStatistic {
+  statName: 'percentileRank';
+}
+
 export interface TrimmedMeanStatistic extends PairStatistic {
   statName: 'trimmedMean';
 }
@@ -154,6 +158,7 @@ export function parseStatistic(
 ):
   | SimpleStatistic
   | PercentileStatistic
+  | PercentileRankStatistic
   | TrimmedMeanStatistic
   | WinsorizedMeanStatistic
   | TrimmedCountStatistic
@@ -188,6 +193,10 @@ export function parseStatistic(
   m = parseSingleStatistic(stat, 'p');
   if (m) return { ...m, statName: 'percentile' } as PercentileStatistic;
 
+  // Percentile Rank statistics
+  m = parsePairStatistic(stat, 'pr');
+  if (m) return { ...m, statName: 'percentileRank' } as PercentileRankStatistic;
+
   // Trimmed mean statistics
   m = parseSingleStatistic(stat, 'tm') || parsePairStatistic(stat, 'tm');
   if (m) return { ...m, statName: 'trimmedMean' } as TrimmedMeanStatistic;

packages/aws-cdk-lib/aws-cloudwatch/test/stats.test.ts

@@ -1,5 +1,20 @@
+import { Metric, Stats } from '../../aws-cloudwatch';
 import * as cloudwatch from '../lib';
 
+it.each([
+  Stats.percentileRank(0),
+  Stats.percentileRank(0, 1),
+  Stats.percentileRank(0, undefined),
+])('Stats can create valid statistics %s without causing warnings', (statistic) => {
+  const metric = new Metric({
+    namespace: 'example',
+    metricName: 'example',
+    statistic,
+  });
+
+  expect(metric.warningsV2).toEqual(undefined);
+});
+
 test('spot check some constants', () => {
   expect(cloudwatch.Stats.AVERAGE).toEqual('Average');
   expect(cloudwatch.Stats.IQM).toEqual('IQM');