DMI supports calculated metrics. In this case, you need total operation time = (average operation time)*(number of operations). Have a look at https://www.dynatrace.com/support/doc/dcrum/short... for more details.
In DC RUM 2018 (now called NAM) calculated metrics are even simpler to manage, so if you can, try it out NAM 2018.
BTW, what is the use case to have this metric?