Function issuer_slot 

pub fn issuer_slot(field: &str) -> String

Where the issuer of a std:oauth2 field is kept, alongside its refresh token: <field key>:std:issuer and <field key>:std:refresh-token.

Namespaced by field key because a credential may hold an OAuth token per upstream, and a compartment keyed by member name alone would have the second overwrite the first — a loss that surfaces only at the first refresh.