Retrieves a list of valid identifiers that can be used as part of a calculated channel expression.
Bearer authentication header of the form Bearer <token>, where <token> is your auth token.
Defaults to 1000. Max of 10,000.
EXPRESSION_MODE_UNSPECIFIED, EXPRESSION_MODE_RULES, EXPRESSION_MODE_CALCULATED_CHANNELS, EXPRESSION_MODE_RULER, EXPRESSION_MODE_STRUCTURED_DATA