[FEATURE] Add label to multichannel mode

[sgofferj]

Is your feature request related to a problem? If so, please describe.
Currently there's only frequency labels in scan mode which makes somewhat sense, however, labels would be useful also for multichannel mode, especially for Prometheus data and when using mixers.

Describe the solution you'd like
Add a "label" directive to channel definitions in multichannel mode. Use this label in Prometheus statistics and send it to Icecast as "Now playing" metadata.

Describe alternative solutions or features you've considered
No alternatives exist.

Additional context
n/a

[charlie-foxtrot]

yep, label is already supported in multi-channel mode but looks like the documentation is missing. Will keep this open until the wiki is updated.