Health checks metrics to CloudWatch

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**Tell us about your request**
Requesting to have health checks metrics and for them to be published to CloudWatch. Enables us to have possibilities to configure SNS, alarms and triggers in response to health checks failures.

**Describe alternatives you've considered**
Its possible to use `4xxStatusResponses` or `5xxStatusResponses` [metrics](https://docs.aws.amazon.com/apprunner/latest/dg/monitor-cw.html?icmpid=docs_apprunner_console#monitor-cw.metrics)