Site24x7 is a monitoring tool and we collect those metrics that are needed for Availability and Performance monitoring of resources. We are GDPR complaint. Based on your question, I am sure you must have gone through our GDPR documents. I am listing some of them for reference.
And, here is a help document link that talks about our server agents and its security measures.
Basically, all the PII data are encrypted by default. Even in your example, for windows event logs, we only collect the event type and count and in case of a failure, the event message is also collected for RCA. That message is encrypted and stored. Such care is taken for all our monitor types too. Wherever we sense the possibility of PII as defined by GDPR, we do take care to encrypt the data at rest.
We are in the process of coming up with a single consolidated document with all these monitor level information and we shall share it in a week's time.