Add support for including plugin output size as metric?

[atc0005]

I'm not sure if it would be useful, but maybe provide an opt-in metric for listing the plugin output size?

Since the metric itself would occupy some of that space, it would need to be added as the very last step affecting plugin output.

The output size calculation might need to be performed on a copy of the plugin output (or via a similar trick).

This might not be all that useful aside from troubleshooting purposes?