cloudwatch: make metricName field public

### Describe the feature

In the following code:
```ts
const sigkillMetricFilter = new MetricFilter(this, 'SigkillMetricFilter', {
filterPattern: FilterPattern.literal('SIGKILL'),
logGroup: this.logGroup,
metricNamespace,
metricName: 'SigkillCount',
metricValue: '1',
});

// Create a metric using the filter
const sigkillMetric = new Metric({
namespace: metricNamespace,
metricName: 'SigkillCount',
statistic: 'Sum',
period: Duration.minutes(5),
});
```

the metricName field is duplicated in two constructs, because it's not possible to do:
```ts
const sigkillMetric = new Metric({
namespace: metricNamespace,
metricName: sigkillMetricFilter.metricName, // this field is private
statistic: 'Sum',
period: Duration.minutes(5),
});
```

This doesn't allow for proper object dependency/relationship.

### Use Case

As above.

### Proposed Solution

Make the field public.

### Other Information

_No response_

### Acknowledgements

- [x] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

aws-cdk-lib@2.180.0

### Environment details (OS name and version, etc.)

MacOS

Start at the TypeScript MetricFilter construct shown in the issue and inspect how metricName is declared and covered by existing tests. Make the property accessible to consumers without changing its value, then run the relevant CloudWatch or MetricFilter tests and confirm the example can use sigkillMetricFilter.metricName.

Written by the indexing model from the issue text.