Which solution will provide this information with the LEAST change to the application?

Explanations:

This option utilizes a CloudWatch Logs metric filter to extract successful login events from the logs without modifying the application code. By configuring dimensions for user name and client name, it allows for easy tracking of unique users on a daily, weekly, and monthly basis while keeping changes minimal.

This option requires modifying the application logic to directly call the AWS SDK on every successful login. This introduces significant changes to the application and increases the risk of bugs or performance issues. Additionally, it may complicate the licensing implementation.

While this option involves minimal changes to the existing CloudWatch agent configuration, it still requires significant alterations. The CloudWatch agent does not natively support metric extraction based on custom logic from logs; it is typically used for more straightforward metric collection and might require additional setup or third-party tools to achieve the desired outcomes.

This option involves deploying a Lambda function that processes CloudWatch Logs, which requires additional infrastructure and complexity. Although it can capture login events, it introduces latency and complexity in managing the function. This is more invasive than simply configuring a metric filter.