Add fargate cost metrics

Because Fargate tasks are billed per second and we can query the AWS API for the start and end times of all tasks that are created as part of the cluster we should be able to calculate how much our cluster is costing.

Two useful metrics would be:
- How much is the current cluster costing? (per second, minute, hour, etc)
- How much has it cost so far? (How long has it been running, has it scaled up and down in that time, etc)

I did this a little in the [notebook](https://gist.github.com/jacobtomlinson/ee5ba79228e42bcc9975faf0179c3d1a) that inspired this module. Would be good to revisit and clean it up.

_Note: This is not useful when running in EC2 mode as users are billed for the instances rather than the tasks. We could request info on the instances bit this is likely to be much less useful._