[ECS/ContainerInsights] [Dimension]: support for *TaskCount metric via TaskDefinitionFamily dimension

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**Tell us about your request**
We'd like the ability to pull `PendingTaskCount` and `RunningTaskCount` metrics from the **TaskDefinitionFamily** dimension

**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?**
Currently `PendingTaskCount` and `RunningTaskCount` metrics can only be gathered from the **ClusterName** & **ServiceName** dimensions, meaning any Task containers managed by a Lambda are unable to report the number of running or pending containers for that particular Task, as these metrics do not support the **TaskDefinitonFamily** dimension. For cases where people dump Container Insights info into external dashboards for visualization (or use the built-in CloudWatch dashboards), we're totally unable to gather information regarding currently-running or pending containers for these Service-less Tasks.

**Are you currently working around this issue?**
We can't, and it sucks haha

**Additional context**
Nope, I think that's it.

The issue requests AWS ECS Container Insights metrics for the TaskDefinitionFamily dimension, but it names no repository files, tests, or implementation entry point. Start by determining whether this roadmap repository contains any implementation scope; done would mean PendingTaskCount and RunningTaskCount are available for TaskDefinitionFamily, including service-less tasks.

Written by the indexing model from the issue text.