Add it to your app:
heroku buildpacks:add https://github.com/ArcadiaPower/heroku-buildpack-xray
The AWS X-Ray daemon (https://github.com/aws/aws-xray-daemon/tree/master/daemon) will pick up environment variables like AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY and AWS_REGION to configure itself.
If present, will use the env var AWS_XRAY_ROLE_ARN to run the daemon using that role arn.