One can argue that lyrics are not really metadata, but
actually the content and a large chunk of text.
Since the default flag is ExtractEverything, nothing will
change for the default case.
The dolphin temp extractor uses KFileMetaData::ExtractionResult::ExtractMetaData explicitly.
Though in general I agree (and "PlainText" should be better named "FullText").
For baloo-widgets, the display of the lyrics tag is disabled anyways. Displaying a large chunk of text does not really look good in the information panel...