Upsert a single custom attribute identified by name. The name is taken from the URL path (URL-encoded). It must be 1-127 characters, must not contain a slash or any control character; spaces, dots, dashes, colons and unicode are allowed. Names starting with $ are reserved for server-side attributes and are rejected (400).

A string value is stored as plain text. A JSON object or array value is stored as a structured value: it is serialized in canonical form (compact, sorted keys) and the attribute is marked with the json flag, so scripts receive it as a hash map or array and JSON representations return it as an object or array rather than as text.