key property
Key is the bucket: a requested path, a referrer domain ("(direct)" for none or a same-origin one), or a utm_source ("(none)" when absent).
Please note: This property should have been non-nullable! Since the specification file does not include a default value (using the "default:" property), however, the generated source code must fall back to having a nullable type. Consider adding a "default:" property in the specification file to hide this note.
Implementation
String? key;