feat(tracing): allow span processor to keep span reference

[samugi]

Fixes #602

Changes

Adds a get_handle method to Span that allows obtaining an instance of SpanHandle: a structure that shares the same fields as the span, where the SpanData is protected by a Mutex.

This allows holding a reference to a Span, which enables use cases such as span processors keeping track of active spans, as mentioned in the spec.

Example:

impl SpanProcessor for MyProcessor {
    fn on_start(&self, span: &mut Span, _cx: &Context) {
        let handle = span.get_handle();
        // Store handle for later use
    }
}

Notes

First contribution here. The change ended up being a bit more invasive than I would've hoped, if there's a cleaner or more idiomatic way to achieve the goal, I'm very happy to adjust the approach and keep iterating on it. Thanks!

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: samugi / name: Samuele (c22aa5e)

[codecov]

Codecov Report

❌ Patch coverage is 96.40449% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.4%. Comparing base (0948c61) to head (c22aa5e).

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/trace/span.rs	96.1%	15 Missing ⚠️
opentelemetry-sdk/src/trace/span_processor.rs	98.1%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3350     +/-   ##
=======================================
+ Coverage   82.2%   82.4%   +0.2%     
=======================================
  Files        128     128             
  Lines      24510   24875    +365     
=======================================
+ Hits       20154   20511    +357     
- Misses      4356    4364      +8     

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[scottgerring]

Hey @samugi thanks for contributing!

This is a pretty chunky change, and I note a SHOULD in the spec; out of curiosity, is there an external driver you'd like to share - e.g. something downstream that needs this to be implemented?

I haven't looked at it in detail yet, but I'd suggest we likely want a reasonably detailed design discussion here. Paging @cijothomas and @bantonsson who almost certainly have useful input :)

[samugi]

hey @scottgerring , thanks for the feedback!

out of curiosity, is there an external driver you'd like to share - e.g. something downstream that needs this to be implemented?

Not really, I picked the issue from the Tracing SDK Stable milestone. Is the milestone up to date and a good source of contributions or would you recommend focusing elsewhere? Thank you!