Container Insights: publish per-task metrics for ECS Fargate tasks (TaskId dimension)

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Currently, Container Insights for ECS Fargate publishes CloudWatch metrics only at the TaskDefinitionFamily level (aggregated across all running tasks of the same family). There is no way to retrieve CPU, memory, or network metrics for an individual task using the TaskId dimension.

Use case:

We run multiple concurrent Fargate tasks from the same task definition (email migration jobs). Each task processes a different user's mailbox and we need to display per-task resource utilization in our monitoring dashboard. Since all tasks share the same TaskDefinitionFamily, the aggregated metrics are useless for identifying which specific task is consuming resources or experiencing issues.

Current behavior:

Querying GetMetricData with dimensions ClusterName + TaskDefinitionFamily → returns aggregated data across all tasks ✓
Querying GetMetricData with dimensions ClusterName + TaskId → returns no data ✗
Expected behavior:

Container Insights should publish metrics with a TaskId dimension (in addition to the existing TaskDefinitionFamily dimension), so users can query CPU, memory, and network usage for individual Fargate tasks.

Workaround:

We currently divide the aggregated metrics by the number of running tasks, which is an inaccurate approximation when tasks have different workloads.

Impact:

This limitation affects any architecture that runs multiple concurrent Fargate tasks from the same task definition and needs per-task observability — a very common pattern for job queues, batch processing, and parallel workloads.

No repository files, tests, or entry points are named. Start by reviewing the described CloudWatch GetMetricData queries and existing Container Insights metric dimensions; done means individual ECS Fargate CPU, memory, and network metrics can be queried with ClusterName plus TaskId.

Written by the indexing model from the issue text.