Feat. Req: Include tracker in all per-torrent metrics

[varac]

I'd like to map download/upload volume per tracker.
Some per-torrent metrics already include the tracker, i.e.

❯ grep -i ubuntu transmission-exporter.metrics

transmission_torrent_leechers{id="21",name="ubuntu-19.04-desktop-amd64.iso",tracker="http://ipv6.torrent.ubuntu.com:6969"} -1
transmission_torrent_leechers{id="21",name="ubuntu-19.04-desktop-amd64.iso",tracker="http://torrent.ubuntu.com:6969"} 118

transmission_torrent_seeders{id="21",name="ubuntu-19.04-desktop-amd64.iso",tracker="http://ipv6.torrent.ubuntu.com:6969"} -1
transmission_torrent_seeders{id="21",name="ubuntu-19.04-desktop-amd64.iso",tracker="http://torrent.ubuntu.com:6969"} 2336
…

But some not:

transmission_torrent_ratio{id="21",name="ubuntu-19.04-desktop-amd64.iso"} 2.2387
transmission_torrent_download_bytes{id="21",name="ubuntu-19.04-desktop-amd64.iso"} 0
transmission_torrent_upload_bytes{id="21",name="ubuntu-19.04-desktop-amd64.iso"} 0

I'd be very interested in having a tracker field in at least those metrics shown above.