As far as I know you cannot see the metrics such as memory used if you are using sandbox (free cloud version). In licensed version you can see this under OPERATE > Mertics > Trends
Hope this helps!
If you have a licenced node, you can take a look at the metrics. If you are using the Free App, there are example screenshots of what you could expect fromt the metrics.
Write a java action, which gives you a string with memory consumed until the action and memory consumed after the action. The difference should give you the memory consumed for your action particularly.
We use this a lot even in our paid cloud to trace memory consumption per MF or even per activity in a MF if needed.