Inaccurate numbers in the sandwich view

The numbers we show in the callee frames in the sandwich view are inaccurate because of the flame graph node truncation.

It's hard to tell how bad the inaccuracy is; however, it's not uncommon to see that 5-25% is hidden in the other stub:

What's worse is that in many cases, the sought-for function might be removed from part of stack traces completely (e.g., if it's a leaf), thereby not being accounted for in the statistics at all. This is especially harmful when investigating low-level functions (e.g., Go `runtime.mallocgc`).

I expect that for a given function, the callee sub-tree is built with respect to max nodes (16K by default). This is not achievable if the caller/callee subtrees are built from the truncated flame graph. Therefore, the caller/callee trees should be built in the backend, before truncation.

Related: #2729