fix(aws-cloudwatch): metric id should be started with a lowercase letter

fixes #13434

Author: BLasan

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

@@ -257,7 +257,7 @@ export class Alarm extends AlarmBase {
     return dispatchMetric(metric, {
       withStat(stat, conf) {
         self.validateMetricStat(stat, metric);
-
+        const id = 'm_' + stat.metricName
         if (conf.renderingProperties?.label == undefined) {
           return dropUndefined({
             dimensions: stat.dimensions,
@@ -283,7 +283,7 @@ export class Alarm extends AlarmBase {
                 stat: stat.statistic,
                 unit: stat.unitFilter,
               },
-              id: stat.metricName,
+              id: id,
               label: conf.renderingProperties?.label,
               returnData: true,
             } as CfnAlarm.MetricDataQueryProperty,

packages/@aws-cdk/aws-cloudwatch/test/integ.alarm-with-label.expected.json

@@ -5,7 +5,7 @@
             "Properties": {
                 "Metrics": [
                     {
-                        "Id": "Metric",
+                        "Id": "m_Metric",
                         "Label": "Metric [AVG: ${AVG}]",
                         "MetricStat": {
                             "Metric": {
@@ -28,7 +28,7 @@
             "Properties": {
                 "Metrics": [
                     {
-                        "Id": "Metric",
+                        "Id": "m_Metric",
                         "Label": "Metric [AVG: ${AVG}]",
                         "MetricStat": {
                             "Metric": {