str
ID of the promotion. For example, "free gift".
The value must be a UTF-8 encoded string with a length limit
of 128 characters, and match the pattern:
``a-zA-Z][a-zA-Z0-9_]*``. For example, id0LikeThis or
ID_1_LIKE_THIS. Otherwise, an INVALID_ARGUMENT error is
returned.
Corresponds to Google Merchant Center property
promotion_id __.