feat: Enable additional CloudFront real-time metrics @jwadolowski (#162)
## whatAdd additional_metrics_enabled variable (default: false for backwards compatibility) that provisions an aws_cloudfront_monitoring_subscription resource when enabled.
why
Not all CloudFront metrics are enabled by default. Some (4xx/5xx error rates by HTTP status code, origin latency, and cache hit rate) are hidden behind a monitoring subscription. This adds a convenient switch to enable them when needed.
The realtime_metrics_subscription_status value could have been derived from var.additional_metrics_enabled, but that would introduce an unexpected resource change across all existing deployments.
references
N/A