[WIP] add profiling to gRPC

[adtac]

This is very much still in development and is subject to massive changes in the near future. Some of the documentation might be outdated, missing, or wrong; I'll fix that soon. The feature itself is experimental and might change rapidly even after merging.

(This won't even build because I'm using runtime.GoId(), which is a local hack that doesn't exist in actual Go.)

I've squashed about 20 local commits to arrive at these 5-6 logically atomic commits; I still have a copy of those 20 commits in case anyone wants them. I'll add new commits on top of this for now and squash them with the appropriate commits later. Or we can just choose to squash everything into one commit (but I think that would lose some information).

Note: WIP

[adtac]

This can now be closed:

internal: #3158

hooks: #3159

service: #3160

cmd: #3161