Include more total values into heap snapshot.

[polina-c]

At the moment heap snapshot contains heap size, but does not contain rss. So, it is hard to connect numbers exposed to the process with ProcessInfo.currentRss, numbers in memory monitor, and numbers in heap snapshot.

Also, now when external size of objects is not accurate, totals are more needed to get some visibility on external size allocations.

Now devtools heap snapshot shows just two numbers: heap and reachanble.
It would be great to show also rss and external, to be able to compare them between snapshots.

[polina-c]

@bkonyi

[a-siva]

@polina-c it should be possible for you to get that information using vm service while grabbing the heap snapshot and write it into a side file which can be used along with the heap snapshot.
One concern with adding this info into the current heap snapshot is that it would be a breaking change.

[polina-c]

Heap snapshot is an async operation.
If i get total values separately, it may not correspond to the snapshotted state of application. And, actually, may diff significantly.

Why it is breaking change?
Because os serialization?
These values may be nullable and ignored.