[ECS] [request]: Automatically create per Service metrics for ECS Fargate SPOT and OnDemand usage

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**Tell us about your request**
What do you want us to build? At this time, FARGATE_SPOT and OnDemand only allow tracking of usage on a regional basis from
```
Namespace: AWS/Usage
Metric Name: ResourceCount
Type: Resource
Resource: Spot
Service: Fargate
```
It would be useful for tracking purposes, especially for seeing when spot instances go down and for how long, if these were automatically tracked at a per Service level in ECS.

**Which service(s) is this request for?**
This could be Fargate, ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
What outcome are you trying to achieve, ultimately, and why is it hard/impossible to do right now? What is the impact of not having this problem solved? The more details you can provide, the better we'll be able to understand and solve the problem.

At this time the solution that I see for this involves listing all the tasks for a service and then describing each of them in order to check if they are FARGATE or FARGATE_SPOT. This is cumbersome for some users to setup and, if an account is using a large number of services, could run into throttling issues.

**Are you currently working around this issue?**
How are you currently solving this problem?

A cron job with a shell or boto3 script would allow for these metrics to be pushed to Cloudwatch.

Start with the ECS Fargate and AWS/Usage ResourceCount details in the issue, including the FARGATE_SPOT and OnDemand distinction and the current task-listing workaround. Done would mean per-Service metrics are automatically available for tracking usage and spot capacity changes without requiring each user to enumerate tasks or risk throttling.

Written by the indexing model from the issue text.