I was looking through the list of plugins and couldn't find anything readily available for DB2, so that metric isnt available out of the box.
An ActiveGate or OneAgent plugin seems like it would be your best bet to pull this into Dynatrace. I have posted some links below to help you determine if this is something you would like to do.
"If you can install OneAgent on a host, use OneAgent extensions.
If you need to monitor a remote technology where OneAgent installation isn't an option, use ActiveGate extensions."
Essentially you would need to pull that metric from the database server using the plugin and expose the metric to Dynatrace.
Hope this helps.