 kit/metrics/cloudwatch2: fails to build ... again

Cloudwatch2 metrics are failing to build again with this message:
```
$go get -t github.com/go-kit/kit/...
# github.com/go-kit/kit/metrics/cloudwatch2
metrics/cloudwatch2/cloudwatch2.go:35:24: undefined: cloudwatchiface.CloudWatchAPI
```
This is due to an update in the the cloudwatch api: https://github.com/aws/aws-sdk-go-v2/pull/298