What is the proper way to pass custom metadata to PhotogrammetrySample?

Which of these would work/be best practice?

sample.metadata = ["FocalLengthIn35mmFilm": "28mm"]
sample.metadata = ["FocalLengthIn35mmFilm": 28.0]
sample.metadata = ["kCGImagePropertyExifFocalLenIn35mmFilm": "28MM"]
What is the proper way to pass custom metadata to PhotogrammetrySample?
 
 
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