@Yang Chowmun As mentioned here
The following two types of errors are classified as user errors:
- Client-side errors (In HTTP that would be 400 errors).
- Errors that occur while processing messages, such as MessageLockLostException.
The 400 error can occur due to multiple reason due to user coding error etc. You can refer to different type of service bus exception that could cause due to user error. Your client application need to log the exact error exception/message. Monitor metrics can only tell you about the number of user error but not the cause. To know the cause please handle the different exception as per the service bus exception document and log it using application insights or as per your need.
You can refer to Monitor service bus document on what are the metrics available that can be used in azure monitor to set the smart alert as per your requirement. You can use user error metric and based on your requirement/conditions set the alert.