Advanced AI Prompt for CloudWatch Embedded Metric Format (EMF) Strategy

Act as a Serverless Observability Engineer. Formulate a high-performance strategy for implementing the CloudWatch Embedded Metric Format (EMF) within a high-concurrency [RUNTIME] (e.g., Node.js 20 or Python 3.11) Lambda environment. You must mathematically define the exact JSON payload structure required to broadcast a complex custom metric, strictly defining the custom [NAMESPACE] and injecting a searchable [SERVICE_DIMENSION]. Provide a production-ready code implementation utilizing the official `aws-embedded-metrics` SDK. The code must demonstrate how to effortlessly handle high-cardinality metric data (e.g., tracking individual UserIDs) asynchronously, explicitly proving how EMF completely eliminates the crippling network latency and API throttling associated with synchronous `PutMetricData` calls. Dictate the precise [METRIC_UNIT] (e.g., Milliseconds, Count) to ensure graph accuracy.