Add metrics for internal transfers

When looking at prometheus metrics it would be helpful to see how much transfer is going on. We have overall network traffic, but this includes communication with external services, like S3.

We already track network transfers internally with `Worker.transfer_{incoming,outgoing}_bytes`, but this is ephemeral and accounts for the amount currently in progress. One way to handle the total would be to add new `Worker.transfer_{incoming,outgoing}_bytes_total` values, and only add to them, rather than also subtract from them.

cc @fjetter , thoughts?

cc @ntabris @dchudz