One could argue that analytics purpose is not direct marketing purpose.
My understanding is that as analytics can be considered as a usual/expected business process, it may use legitimate interests as far as it fulfill requirements (information of the process, the right to opt-out, ...).
However, the problem is that analytics may be advanced analytics. Is the retrieval of Adwords parameters from a glcid allowed/expected ? Is the injection of historical behaviour or marketing segment allowed/expected ?
"The processing of personal data for direct marketing purposes may be regarded as carried out for a legitimate interest."
It's also mentioned in Article 21 describing the right to object to processing using legitimate/public interest:
"Where personal data are processed for direct marketing purposes, the data subject shall have the right to object at any time… etc."
The ICO has some useful guidance on when it is an appropriate basis: https://ico.org.uk/for-organisations/guide-to-data-protectio...