[Feature request] Display bytes transferred when updating <crates.io> index

[gendx]

Describe the problem you are trying to solve
When updating an index such as crates.io on a slow network connection (e.g. tethering from a mobile device), the "Fetch" step is often quite slow. In particular on a fresh install of Cargo this is often the slowest step.

For now, there is a progress bar showing a percentage of progress, such as the following.

      Fetch [============================>                          ]  53.00%

However, this percentage is quite opaque.

Describe the solution you'd like
It would be useful to also display:

1. how many bytes have been received,
2. how many bytes are remaining,
3. the current throughput.

Notes
I believe the relevant code that updates the progress bar is the following:

cargo/src/cargo/sources/git/utils.rs

Lines 693 to 697 in f12d72e

	rcb.transfer_progress(|stats| {
	progress
	.tick(stats.indexed_objects(), stats.total_objects())
	.is_ok()
	});

In particular, the relevant information (stats) seem to come from a callback from libgit2 (git2::Progress). This struct already contains a received_bytes() which would provide information for (1). I'm not sure how to get the total/remaining bytes from libgit2, nor the throughput. But seeing how many bytes have been transferred would already be a great improvement.

[Eh2406]

That sounds reasonable to me!