Add symbolize feature for online symbolization

[erikgrinaker]

This patch adds a symbolize feature which performs online symbolization of profiles using the backtrace crate.

This is often much more convenient than having to manually obtain binaries and symbolize the profiles, and matches the behavior of e.g. the pprof-rs CPU profiler. It will also allow emitting e.g. flamegraph SVGs directly, which I'll submit in a follow-up PR.

Online symbolization uses a fair amount of memory for symbol caches, but this is often a worthwhile tradeoff.

[CLAassistant]

All committers have signed the CLA.

[erikgrinaker]

Hi @umanwizard, just a friendly ping in case y'all missed this.

[umanwizard]

Thanks and sorry for the delay.

Did you test with stack traces that have inlined functions? Did it work correctly / can you provide an example .pprof ?

[erikgrinaker]

No worries.

Did you test with stack traces that have inlined functions?

No, I haven't tested it. I think it should mostly just work as long as there's sufficient debug info (backtrace supports it), but I'll verify. And we should probably set Mapping::has_inline_frames in that case.

I'll have a look over the weekend and get back to you.

[erikgrinaker]

Confirming that inlined functions work as expected, using:

#[inline(never)]
fn allocate(size: usize) -> Vec<u8> {
    allocate_inline(size)
}

#[inline(always)]
fn allocate_inline(size: usize) -> Vec<u8> {
    println!("allocating {size}b");
    vec![9; size]
}

Which yields profile.pb.gz: