Auto merge of #20626 - avadacatavra:pwm-spec, r=asajeffrey
Auto merge of #20626 - avadacatavra:pwm-spec, r=asajeffrey

added spec comments for progressive web metrics

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r? @asajeffrey

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- [X] `./mach build -d` does not report any errors
- [X] `./mach build-geckolib` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #20624 (github issue number if applicable).

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