[ECS] [request]: Cloudwatch Metrics for ECS Deployments

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**Tell us about your request**
Provide metrics on deployments for an ECS service.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I've searched the `ECS` and `ECS/ContainerInsights` namespaces and don't see any metric for ECS deployments. We have an ECS-hosted legacy application that unfortunately needs to be bounced now and again - simply forcing a 100/200 deployment of the same taskdef works great.

However, on our monitoring dashboards, the only way we get an indication that this has happened is seeing that the `DesiredTaskCount` has doubled briefly, then returned to normal. During load, or if the service is behaving in a degraded manner, I would love to be able to tell if such task count changes are due to service autoscaling/degradation, if someone modified the service task counts, or if a deployment happened.

It would be great to see a `ActiveDeploymentCount` metric. (A `DeploymentDuration` metric would be nice too, but that's less important).

**Are you currently working around this issue?**
Right now we just dig into the ECS events tab in the UI. We _could_ update the deployment scripts to push a custom metric during deployment, but it wouldn't help when someone forces an update via the console UI.

No repository files or tests are named. Start by reviewing the ECS and ECS/ContainerInsights metric namespaces and the ECS events behavior described in the issue; done would mean ECS exposes an ActiveDeploymentCount metric, with DeploymentDuration as an optional addition.

Written by the indexing model from the issue text.