Update ECS Fargate about potential billing impact related to setting …

…DD_HOSTNAME (#18031)

* added section to enable default metrics through jmxfetch

* update dd_hostname warning to include note about billing impact

* Update ecs_fargate/README.md

Co-authored-by: Alicia Scott <aliciascott@users.noreply.github.com>

---------

Co-authored-by: Alicia Scott <aliciascott@users.noreply.github.com>

ecs_fargate/README.md

@@ -297,7 +297,7 @@ CloudFormation example (YAML):
               Value: "{\"com.docker.compose.service\":\"service_name\"}"
 ```
 
-**Note**: You should not use `DD_HOSTNAME` since there is no concept of a host to the user in Fargate. `DD_TAGS` is traditionally used to assign host tags, but as of Datadog Agent version 6.13.0 you can also use the environment variable to set global tags on your integration metrics.
+**Note**: You should not use `DD_HOSTNAME` since there is no concept of a host to the user in Fargate. Using this tag can cause your tasks to appear as APM Hosts in the Infrastructure list, potentially impacting your billing. Instead, `DD_TAGS` is traditionally used to assign host tags. As of Datadog Agent version 6.13.0, you can also use the `DD_TAGS` environment variable to set global tags on your integration metrics.
 
 ### Crawler-based metrics