Function canonical 

pub fn canonical(key: &str) -> Option<&'static str>

Returns the “canonical” version of an attribute. This means:

• If the attribute is not deprecated, it is returned itself.
• If the attribute is deprecated and has a replacement, the replacement is returned.
• If the attribute is deprecated without a replacement, None is returned.
• If the attribute is not defined in sentry-conventions, None is returned.