Pena Javier - Welcome to Microsoft Q&A and thanks for reaching out to us. As far as i know, it is not possible to dynamically include log analytics data in the email subject field of an alert notification in Azure Monitor. The email subject field can only be customized with static text.
However, you can use the webhook action to send the alert data to an external system, such as a Logic App, where you can further process the data and customize the email subject field as needed.
Hope this helps. and please feel free to reach out if you have any further questions.
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