[ECS] [request]: Allow multiple service discovery registry entries for a single service

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**Tell us about your request**
I'd like to have a single ecs service that exposes 2 ports and have both registered in service discovery registries.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We're using the fluentbit/firelens setup for our logs, fluentbit exposes prometheus metrics by default and with the dns_sd config in prometheus we can really easily get our application metrics into prometheus (very nice), but we can't use that same mechanism to get metrics from fluentbit.

This is our immediate goal but I would have thought this would be a common concern for anyone making use of the service discover feature.

**Are you currently working around this issue?**
No, we're not collecting any metrics from sidecars.

**Additional context**
I understand that we can run consul to solve this problem but having the whole solution provisioned with a few lines of terraform is very attractive.

Start by reviewing the ECS service request and its service-discovery context, including the two-port and sidecar metrics use case described here. Done means a single ECS service can expose two ports and register both in service discovery registries, while remaining provisionable with Terraform.

Written by the indexing model from the issue text.