Hi,
this form of static threshold alerting is currently not supported as an in product alert. It is technically possible to achieve by writing a script against out log query API.
This functionality is on our radar and we plan to deliver it as an in product feature, hopefully some time next year.
Here is an older post which asked a similar question Question on log search - #6 by david_smith
If you are willing and able to write scripts against the API, I’m happy to share a working example which could be used as a suggestion to tweak and suit your needs. As I mentioned in that previous post.
David