What are you trying to get? Where to scrape the metrics from your containerized applications or to where to scrape ECS-specific metrics?
The latter - for example: desired containers or CPU utilization by an ECS service - are only exposed through AWS CloudWatch and there’s no other option than to use a cloudwatch exporter.
What the Prometheus ECS Discovery plugin will be helpful is to be able to dynamically find where to scrape the /metrics endpoint for your apps that expose Prometheus metrics.
I haven’t used it, but I know there can be some wrinkles to iron if the number of containers you run in your environment is really large or it changes really frequently.