Tracking of memory used by in-memory index

[dslizardo]

This question was originally raised at #bluge channel in Gophers slack and was told by @mschoch to open an issue here.

We're using Bluge in-memory index and we want to know how much memory our index using. As per our previous conversation this feature is not quite well supported today. Existing stats are focused on disk-usage.

It would be nice to have a way to track memory usage of in-memory index.