Attach `tgid` as pprof label to samples

[brancz]

Is your feature request related to a problem? Please describe.

It can be useful to disambiguate threads with the same stack for example to understand whether worker pools are being appropriately utilized and balanced.

Describe the solution you'd like

Attach the tgid label to samples.

Describe alternatives you've considered

I suppose it could be a target label, but that doesn't make sense to me as to me target label sets are there to identify the process uniquely and intentionally stop there.

[brancz]

Since we already disambiguate stacks using the tgid and we already communicate it from the eBPF program to user-space I think this should only be a matter of setting it in the resulting sample.