Parallelize candidate lookup for metadata plugins.

[semohr]

This pull request introduces concurrent execution of metadata source plugin searches and lookups, which significantly improves performance when multiple plugins are enabled. Instead of running each plugin sequentially, the code now uses threads to perform plugin lookups in parallel, reducing overall wait time for I/O-bound operations.

For me this improved lookup times by up to 5s each! The improvement should roughly scale linear with the number of enabled metadata plugins.

[Copilot]

Pull request overview

PR make metadata source plugin search/lookup run in parallel threads, so total wait time go down when many plugins enabled.

Changes:

• Run candidates / item_candidates / albums_for_ids / tracks_for_ids across metadata source plugins using ThreadPoolExecutor.
• Add changelog entry for concurrent metadata plugin execution.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
docs/changelog.rst	Document new concurrent metadata plugin searches/lookups feature.
beets/metadata_plugins.py	Replace sequential plugin iteration with threaded execution using futures.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.19%. Comparing base (be33782) to head (f7ed69b).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6546   +/-   ##
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  Coverage   71.18%   71.19%           
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  Files         150      150           
  Lines       19172    19175    +3     
  Branches     3084     3084           
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+ Hits        13648    13651    +3     
  Misses       4864     4864           
  Partials      660      660           

Files with missing lines	Coverage Δ
beets/metadata_plugins.py	84.47% <100.00%> (+0.29%)	⬆️

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[snejus]

squinting-skeptical.mp4

Or maybe it's just our nice design 😆

[semohr]

@snejus We should consider doing something similar for send as the events handling could also be parallelized. For me the event handling takes more time than the lookup itself :/