[Fargate] [Metrics]: Container Insights Duration Metric

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**Tell us about your request**
It would be great to be able to have metrics to know how long a task runs for. Currently, for Lambda there is a `Duration` metric.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Knowing how long a task typically takes can be used as a benchmark for code improvements. This ensures the changes are moving in the right direction.

**Are you currently working around this issue?**
We're logging the total duration to CloudWatch Logs then pulling it out of the logs manually.

Review the Fargate and Container Insights service documentation first, since the issue names no repository files, tests, or implementation entry points. Confirm the requested task-duration metric's scope and acceptance criteria with maintainers; done would mean the metric is defined and available for Fargate tasks.

Written by the indexing model from the issue text.