[ECS] [request]: Expose container memory reservation in the metadata stats endpoint

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**Tell us about your request**
Please expose the container memory reservation in the container stats endpoint

**Which service(s) is this request for?**
ECS (EC2)

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I am trying to tune the resource allocation of our ECS deployments. To do this, I need to understand resource consumption, any limits and any reservations which are configured. Currently (unless I'm mistaken) the memory reservation for a container in a task is only available either via `docker inspect` under `MemoryReservation` or in the memory cgroup settings. It would be much more convenient to have this information available alongside the other container statistics in the stats endpoint.

**Are you currently working around this issue?**
We are using the cloudwatch `MemoryUtilization` and `CPUUtilization` metrics for ECS services, but these do not go down to container level (container insights probably does - I haven't checked yet). We are also running telegraf as a sidecar in each ECS task to expose the task container stats to Prometheus, which allows us to track resource usage per-container (and limits per-container).

No repository file, test, or implementation entry point is identified. Start by confirming the ECS (EC2) container stats endpoint and how Docker's MemoryReservation or cgroup value maps to the requested field; done means that reservation is exposed alongside the existing per-container statistics.

Written by the indexing model from the issue text.