replaygain: add metaflac backend

[SamadBallaj1]

Closes #1203

The replaygain plugin can't compute ReplayGain when metaflac is the only tool available. The existing backends (command, gstreamer, audiotools, ffmpeg) each need something heavier installed, which doesn't work on minimal setups like a NAS where those won't install. Issue #1203 asks for a metaflac based option so FLAC users on those setups can still tag ReplayGain.

This adds a metaflac backend for that case. It runs metaflac --add-replay-gain to compute the gain and reads the values back with metaflac --show-tag. I modeled it on the existing CommandBackend since both wrap an external tool. It only handles FLAC, skips other formats, and shifts the gain to the configured targetlevel like the other backends.

One limitation: metaflac scans a whole album in one pass, so the files of an album need the same sample rate and channel layout.

Before, selecting the metaflac backend failed:

$ beet replaygain
UserError: Selected ReplayGain backend metaflac is not supported.

After, the backend works and the plugin tests pass:

$ python -m pytest test/plugins/test_replaygain.py
7 passed, 4 skipped

The 4 skipped are the Opus R128 cases, which don't apply to the metaflac path.

To Do

• Documentation.
• Changelog.
• Tests.

[codecov]

Codecov Report

❌ Patch coverage is 83.92857% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.47%. Comparing base (0fd0e82) to head (2d354c2).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/replaygain.py	83.92%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6800      +/-   ##
==========================================
+ Coverage   74.88%   75.47%   +0.58%     
==========================================
  Files         163      163              
  Lines       20998    21054      +56     
  Branches     3307     3312       +5     
==========================================
+ Hits        15725    15890     +165     
+ Misses       4519     4393     -126     
- Partials      754      771      +17     

Files with missing lines	Coverage Δ
beetsplug/replaygain.py	46.58% <83.92%> (+20.25%)	⬆️

... and 1 file with indirect coverage changes

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[SamadBallaj1]

Pushed some updates, this is ready for review whenever you have a chance. Thanks.

[snejus]

Looks good, have you tested it yourself?

[snejus]

Suggested change

	for item in task.items:
	if self.format_supported(item):
	for item in filter(self.format_supported, task.items):

[SamadBallaj1]

Yes. I ran it locally on a FLAC file with the metaflac backend and it wrote the correct ReplayGain tags (track gain got arund -11.5 dB, peak close to 1.0). The metaflac tests also pass on my machine.

[snejus]

Would you mind installing flac in this part of the workflow, in ci.yaml?

      - name: Install system dependencies on Ubuntu

[SamadBallaj1]

Thanks @snejus for the review. I updated the track loop to use filter like you suggested and also added flac to the Ubuntu deps so the metaflac tests run on CI. All clear now, both pushed.