Fix duplicate glyphs3 GSMetrics; take the first and ignore 'filter'

[anthrotype]

GSMetric's filter attribute (that restrict a given metric to a group of glyphs, e.g. small caps) is not relevant to build OS/2 and MVAR tables. Some source contains those, normally following the same-named metric which is supposed to apply globally (and thus be stored in font wide metric tables).
glyphsLib does parse these filter attributes but doesn't even use them, when looking up a metric by name it simply returns the first one it finds. Similarly here when we collect metrics by name, we only take the first one we find and ignore any duplicate that may follow.

Fixes #1269

After the fix, the following gives "output is identical"

$ python resources/scripts/ttx_diff.py 'https://github.com/Etcetera-Type-Co/Epilogue#sources/Epilogue.glyphs' --compare gftools --config ~/.fontc_crater_cache/Etcetera-Type-Co/Epilogue/sources/config.yaml

[anthrotype]

the test reproducer (92b89b3) fails as expected (the last duplicate, the one with the 'filter', prevails)

---- font::tests::v3_duplicate_metrics_first_wins stdout ----
thread 'font::tests::v3_duplicate_metrics_first_wins' panicked at glyphs-reader/src/font.rs:3598:9:
assertion failed: `(left == right)`

Diff < left / right > :
 Some(
     (
         Some(
<            450.0,
>            501.0,
         ),
         None,
     ),
 )

https://github.com/googlefonts/fontc/actions/runs/13329313658/job/37229531501

[cmyr]

looks good!