Add OTEL trace id to goroutine labels

[SimonRichardson]

To better integrate with 3rd party applications[1] and to identify goroutines that are related to OTEL traces, this adds labels to a goroutine.

This is another step forward to better understand the performance characteristics of how Juju operates in theatre.

1. https://www.polarsignals.com/blog/posts/2024/03/05/correlating-tracing-with-profiling-using-ebpf

Checklist

• Code style: imports ordered, good names, simple structure, etc
• Comments saying why design decisions were made
• Go unit tests, with comments saying what you're testing

QA steps

$ sudo snap install parca-agent
$ sudo snap set parca-agent metadata-external-labels=machine=<machine>
$ sudo snap set parca-agent remote-store-bearer-token=<token>
$ juju bootstrap lxd test

See the labels added to the profiles.

[SimonRichardson]

/build

[manadart]

Working well.

[SimonRichardson]

/merge