Support for AWS SDK V2 `MetricsPublisher`

I have an application that uses AWS SDK V2 to perform multiple API calls, and that monitors those API calls using the recently released [metrics module for AWS SDK V2](https://docs.aws.amazon.com/sdk-for-java/v2/developer-guide/configuration-metrics.html)

My application runs on Lambda, and uses EMF to avoid the pitfalls of calling CloudWatch APIs directly to emit monitoring metrics. Using this library we've been able to replace all of our CloudWatch API calls, except for the metrics emitted by the AWS SDK V2.

Because AWS' recommendation for metric tracking on ephemeral containers is to use EMF, it would be good to have integration in this library with the `MetricsPublisher` interface that the AWS SDK V2 exposes, so that we can monitor our AWS SDK calls with EMF as well.

I'm currently interested in Lambda support, but support for containers would also be good.