/usr/share/fish/completions/apt-key.fish is in fish-common 2.4.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | #apt-key
complete -r -c apt-key -a add --description "Add a new key"
complete -f -c apt-key -a del --description "Remove a key"
complete -f -c apt-key -a list --description "List trusted keys"
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