add more runtime metrics to activation records?

This is an open-ended question about the possibility of adding more runtime metrics to the activation records, like #2846 did.

I've been working on a [metrics library](https://github.com/jthomas/openwhisk-metrics) for openwhisk that records runtime metrics using a Node.js function proxy. If is was possible for the platform to include these values directly in the activation records, it would negate the need for a separate library and support all runtimes.

This would be dependent on the values exposed by [docker stats API](https://docs.docker.com/engine/api/v1.33/#operation/ContainerStats)?

Metric values that would be useful for users include...
- Cold start (I know this can be inferred from missing `initTime` but might be nice to have a convenience value).
- Memory usage including OOM errors?
- CPU Usage