Update the setting guideline to get ecs.containerinsights metrics

[aquiladayc]

Documentation Page

Public doc: https://docs.datadoghq.com/integrations/amazon_ecs/#metrics

Note: Metrics prefixed with ecs.containerinsights.* come from the AWS CloudWatch agent.

Source code: https://github.com/DataDog/dogweb/blob/prod/integration/amazon_ecs/README.md

Issue Description

AWS CloudWatch agent is not very clear and the link is led to AWS integration tile but not explained what end-user needs to do.
Required setting is enable Collect custom metrics in AWS integration tile.