Prometheus: report bytes remaining

[Alan Somers]

For recently added jobs that haven't finished a full initial replication, it would be useful to report how much data remains to transfer. For example, if zrepl status shows this:

Progress: [\--------------------------------------------------] 252.6 GiB / 22.3 TiB @ 121.0 MiB/s (2d  5h  1m 28s remaining)
* foo  STEPPING (step 1/25, 16.2 GiB/3.3 TiB) next: full send @zrepl_foo20240211_100807_000 (resumed)
* bar  STEPPING (step 1/23, 236.4 GiB/19.0 TiB) next: full send @zrepl_bar20240211_100807_000 (resumed)

Then it would be useful for the Prometheus exporter to report something like this:

zrepl_replication_bytes_remaining{filesystem="foo",zrepl_job="myjob"} 3.61099e+12
zrepl_replication_bytes_remaining{filesystem="bar",zrepl_job="myjob"} 2.06369e+13