I'm not sure if you are aware, but you have a view in Dynatrace where you can filter the exception and chart it over time, but this view won't be refreshed and you can't pin it to a dashboard as a dashlet.
Go to the specific service -> "View dynamic requests" -> "View exception analysis" -> filter by the exception you want.
You use a split option ?
Could you please show the configurator of your Custom metric?
Why you take Average as Aggregation and not Sum or count ?
Also, if there any way to filter by the message of each Exception?
I want also to make another metric base on response time splited by Message.
Have a good day.