Record backup size estimate in backup metadata

[infogulch]

I suspected some of my apps were consuming a lot of space recently, and I had to do a fair amount of digging before the details became evident. To me this was particularly concerning for backups because they don't do any deduplication and I could end up with many multiples of this increased disk consumption.

It would be nice to have the size of each backup recorded in the metadata for backups. Even if it's a little off compared to my backup host it would still be nice to have an order-of-magnitude estimate.