AWS Lambda's Event-Source Mapping (ESM) has improved its functionality with the introduction of new metrics, providing users with visibility into its working and helping them monitor performance and troubleshoot issues. These metrics give users insights into the number of events ESM successfully polled from the source, the number of events sent to the target Lambda function, and other key metrics such as deleted events, dropped events, and failed invocations for supported sources like SQS, Kinesis, and DynamoDB. The new metrics are charged at CloudWatch's standard rate of $0.30 per metric per month, with no charge for PutMetric API calls. Users can enable these metrics through various AWS services, including CloudFormation, AWS SDK, AWS CLI, and the AWS console, and can use tools like Lumigo to create alerts and monitor their event sources.