Include taskId / instanceId in metric dimensions

We're trying this wonderful gem out in our ECS-deployed application. We have a number of copies of the task running in the service, and it would useful to have a taskId dimension available.

Because it would be tricky to specify the taskId an ECS instance variable, it could be a cool idea to automatically include taskId as an dimension when the puma process is detected as running on ECS, and similar for EC2 instance ID. I'm happy to have a go implementing, but I'm wondering if it's something you'd be interested in accepting, before I do so.

Start by tracing how puma-cloudwatch currently constructs metric dimensions and review the ECS and EC2 metadata available to the running process. Define the accepted behavior for ECS task IDs and EC2 instance IDs, including when they are unavailable, then verify the resulting CloudWatch dimensions in the relevant metric output or tests.

Written by the indexing model from the issue text.