chore(scheduler-alpha): increase unit test coverage (#32033)

### Issue # (if applicable)

tracking issue: #31785 

### Reason for this change

Satisfy 90% unit test coverage required for developer preview (see https://github.com/cdklabs/team-internal/blob/main/docs/construct-library-lifecycle.md#exit-criteria-1)

### Description of changes

Added parameterized unit tests for metric methods

Previous coverage summary:
```
=============================== Coverage summary ===============================
Statements   : 83.06% ( 103/124 )
Branches     : 76.19% ( 32/42 )
Functions    : 70.9% ( 39/55 )
Lines        : 83.47% ( 101/121 )
================================================================================
```
Current coverage summary:

```
=============================== Coverage summary ===============================
Statements   : 95.16% ( 118/124 )
Branches     : 83.33% ( 35/42 )
Functions    : 96.36% ( 53/55 )
Lines        : 95.86% ( 116/121 )
================================================================================
```

### Description of how you validated changes

Unit tests pass

### Checklist
- [x] 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: gracelu0

packages/@aws-cdk/aws-scheduler-alpha/lib/group.ts

@@ -202,7 +202,7 @@ abstract class GroupBase extends Resource implements IGroup {
    *
    * @default - sum over 5 minutes
    */
-  metricSentToDLQ(props?: cloudwatch.MetricOptions): cloudwatch.Metric {
+  public metricSentToDLQ(props?: cloudwatch.MetricOptions): cloudwatch.Metric {
     return this.metric('InvocationsSentToDeadLetterCount', props);
   }
 

packages/@aws-cdk/aws-scheduler-alpha/test/group.test.ts

@@ -279,20 +279,33 @@ describe('Schedule Group', () => {
       },
     });
   });
+});
 
-  test('Target Error Metrics', () => {
+describe('Schedule Group Metrics', () => {
+  test.each([
+    ['metricTargetErrors', 'TargetErrorCount'],
+    ['metricThrottled', 'InvocationThrottleCount'],
+    ['metricAttempts', 'InvocationAttemptCount'],
+    ['metricTargetThrottled', 'TargetErrorThrottledCount'],
+    ['metricDropped', 'InvocationDroppedCount'],
+    ['metricSentToDLQ', 'InvocationsSentToDeadLetterCount'],
+    ['metricSentToDLQTruncated', 'InvocationsSentToDeadLetterCount_Truncated_MessageSizeExceeded'],
+  ])('calling %s creates alarm for %s metric', (metricMethodName, metricName) => {
     // GIVEN
+    const app = new App();
     const props: GroupProps = {
       groupName: 'MyGroup',
     };
+    const stack = new Stack(app, 'Stack', { env: { region: 'us-east-1', account: '123456789012' } });
     const group = new Group(stack, 'TestGroup', props);
 
     // WHEN
-    const metricTargetErrors = group.metricTargetErrors({
+    const metricMethod = (group as any)[metricMethodName].bind(group); // Get the method dynamically
+    const metricTargetErrors = metricMethod({
       period: Duration.minutes(1),
     });
 
-    new cw.Alarm(stack, 'GroupTargetErrorAlarm', {
+    new cw.Alarm(stack, `Group${metricName}Alarm`, {
       metric: metricTargetErrors,
       evaluationPeriods: 1,
       threshold: 1,
@@ -306,7 +319,41 @@ describe('Schedule Group', () => {
           Value: 'MyGroup',
         }),
       ]),
-      MetricName: 'TargetErrorCount',
+      MetricName: metricName,
+      Namespace: 'AWS/Scheduler',
+    });
+  });
+
+  test('Invocations Failed to Deliver to DLQ Metrics', () => {
+    // GIVEN
+    const app = new App();
+    const props: GroupProps = {
+      groupName: 'MyGroup',
+    };
+    const stack = new Stack(app, 'Stack', { env: { region: 'us-east-1', account: '123456789012' } });
+    const group = new Group(stack, 'TestGroup', props);
+    const errorCode = '403';
+
+    // WHEN
+    const metricFailedToBeSentToDLQ = group.metricFailedToBeSentToDLQ(errorCode, {
+      period: Duration.minutes(1),
+    });
+
+    new cw.Alarm(stack, 'GroupFailedInvocationsToDLQAlarm', {
+      metric: metricFailedToBeSentToDLQ,
+      evaluationPeriods: 1,
+      threshold: 1,
+    });
+
+    // THEN
+    Template.fromStack(stack).hasResourceProperties('AWS::CloudWatch::Alarm', {
+      Dimensions: Match.arrayWith([
+        Match.objectLike({
+          Name: 'ScheduleGroup',
+          Value: 'MyGroup',
+        }),
+      ]),
+      MetricName: `InvocationsFailedToBeSentToDeadLetterCount_${errorCode}`,
       Namespace: 'AWS/Scheduler',
     });
   });

packages/@aws-cdk/aws-scheduler-alpha/test/schedule-expression.test.ts

@@ -118,7 +118,7 @@ describe('schedule expression', () => {
   });
 
   test('one-time expression with invalid date throws', () => {
-    expect(() => ScheduleExpression.at(new Date('13-20-1969'))).toThrowError('Invalid date');
+    expect(() => ScheduleExpression.at(new Date('13-20-1969'))).toThrow('Invalid date');
   });
 });
 
@@ -130,13 +130,13 @@ describe('fractional minutes checks', () => {
   });
 
   test('rate cannot be a fractional amount of minutes (defined with minutes)', () => {
-    expect(()=> {
-      ScheduleExpression.rate(Duration.minutes(5/3));
+    expect(() => {
+      ScheduleExpression.rate(Duration.minutes(5 / 3));
     }).toThrow(/must be a whole number of/);
   });
 
   test('rate cannot be a fractional amount of minutes (defined with hours)', () => {
-    expect(()=> {
+    expect(() => {
       ScheduleExpression.rate(Duration.hours(1.03));
     }).toThrow(/cannot be converted into a whole number of/);
   });
@@ -149,7 +149,7 @@ describe('fractional minutes checks', () => {
 
   test('rate cannot be less than 1 minute (defined with minutes as fractions)', () => {
     expect(() => {
-      ScheduleExpression.rate(Duration.minutes(1/2));
+      ScheduleExpression.rate(Duration.minutes(1 / 2));
     }).toThrow(/must be a whole number of/);
   });
 
@@ -164,4 +164,9 @@ describe('fractional minutes checks', () => {
       ScheduleExpression.rate(Duration.minutes(10))
         .expressionString);
   });
-});
+
+  test('literal schedule expression', () => {
+    expect('rate(1 hour)').toEqual(
+      ScheduleExpression.expression('rate(1 hour)').expressionString);
+  });
+});

packages/@aws-cdk/aws-scheduler-alpha/test/schedule.test.ts

@@ -64,13 +64,22 @@ describe('Schedule', () => {
     });
   });
 
-  test('returns metric for delivery of failed invocations to DLQ', () => {
+  test.each([
+    ['metricAllThrottled', 'InvocationThrottleCount'],
+    ['metricAllErrors', 'TargetErrorCount'],
+    ['metricAllAttempts', 'InvocationAttemptCount'],
+    ['metricAllTargetThrottled', 'TargetErrorThrottledCount'],
+    ['metricAllDropped', 'InvocationDroppedCount'],
+    ['metricAllSentToDLQ', 'InvocationsSentToDeadLetterCount'],
+    ['metricAllSentToDLQTruncated', 'InvocationsSentToDeadLetterCount_Truncated_MessageSizeExceeded'],
+
+  ])('returns expected metric for %s', (metricMethodName: string, metricName: string) => {
     // WHEN
-    const metric = Schedule.metricAllFailedToBeSentToDLQ();
+    const metric = (Schedule as any)[metricMethodName]();
 
     // THEN
     expect(metric.namespace).toEqual('AWS/Scheduler');
-    expect(metric.metricName).toEqual('InvocationsFailedToBeSentToDeadLetterCount');
+    expect(metric.metricName).toEqual(metricName);
     expect(metric.dimensions).toBeUndefined();
     expect(metric.statistic).toEqual('Sum');
     expect(metric.period).toEqual(Duration.minutes(5));
@@ -88,6 +97,18 @@ describe('Schedule', () => {
     expect(metric.period).toEqual(Duration.minutes(5));
   });
 
+  test('returns metric for delivery of failed invocations to DLQ with no error code', () => {
+    // WHEN
+    const metric = Schedule.metricAllFailedToBeSentToDLQ();
+
+    // THEN
+    expect(metric.namespace).toEqual('AWS/Scheduler');
+    expect(metric.metricName).toEqual('InvocationsFailedToBeSentToDeadLetterCount');
+    expect(metric.dimensions).toBeUndefined();
+    expect(metric.statistic).toEqual('Sum');
+    expect(metric.period).toEqual(Duration.minutes(5));
+  });
+
   test('returns metric for all errors with provided statistic and period', () => {
     // WHEN
     const metric = Schedule.metricAllErrors({