nix copy should show progress

[lilyball]

Is your feature request related to a problem? Please describe.
When I'm deploying a configuration to my remote machine with nix copy, it tells me what path it's currently copying, but it doesn't tell me anything else about its progress. I don't know how many paths it has left to copy, how much data it's copying, or how far along in the copy it is.

Describe the solution you'd like
nix copy should show some indication of how many paths it has to copy and how many it has already copied, kind of like how nix build shows number of derivations to build and how many it has already built.

Ideally it would also show some indication of the size of the copied data (e.g. total size of all closures to copy, size already copied) like how nix build shows stats on downloaded paths. Even nicer if it can show me the rate at which data is being copied and estimate how long it will take to finish.

[rapenne-s]

It seems ok from my point of view?

nix copy --to file:///tmp/firefox /nix/store/5rh31h7inyiac57dvggwd0v8hka1qpdw-firefox-unwrapped-101.0.1/

gives a progress line like all flakes command looking like

[1/141/150 copied (0.0/631.3 MiB)] copying path '/nix/store/h8gadrivdrl2rh71v38ly22ihmffl6k0-systemd-250.4' to 'file:///tmp/firefox'

[wamserma]

gives a progress line like all flakes command looking like

[1/141/150 copied (0.0/631.3 MiB)] copying path '/nix/store/h8gadrivdrl2rh71v38ly22ihmffl6k0-systemd-250.4' to 'file:///tmp/firefox'

But this progress is not updating properly. In your example 141 paths have been copied, but it still says "0.0" of "631.3 MiB".

Similar here with Nix 2.11.1:
[4/633/1481 copied (0.0/6934.9 MiB)] copying path '/nix/store/np4sh6in90c2yjm5znfr1qcaznlmyj7l-llvm-14.0.6-lib' to 'ssh://xxx@XXX'

[NobbZ]

Probably related #8108?