test : Add test to confirm Context expected behavior

[cijothomas]

To assist reviewing https://github.com/open-telemetry/opentelemetry-rust/pull/2378/files I wrote some tests, which left me wondering with a unresolved question.

Context MUST be immutable, so adding value to context (with_value), should result in a new Context, leaving original untouched. A test for this is added and works as expected.
Then I tried this with Spans - Created an empty context, and added a Span to the Context. When I add another random value to that Context, it is cloning the random value, but the Span is stored via Arc, so clone is just pointing to the original span. Updates made to span from the new context updates the original span.
Not sure if this is the intended behavior, opening to discuss.

Edit: The expectation is Context itself is immutable, but the Span inside is mutable. Tests modified to assert this.

[scottgerring]

I used this as an excuse to have a read of the spec 😄 My read is that the Span can and is designed to mutate internally without the surrounding Context caring.

A Context is immutable: Set Value operation

The API MUST return a new Context containing the new value.

Also, A SpanContext is immutable, but this doesn't matter much for the present discussion:

SpanContexts are immutable.

Whereas, Spans are very mutable - "Set Attributes", "Add Events", etc.

None of these operations nor the section Context Interaction mention anything about span mutation forcing a corresponding update/replace on the context. This would be in principal quite strange 2-way coupling - the Span having to tell the thing that happens to contain it that it had changed.

[cijothomas]

@bantonsson @scottgerring Thanks for clearing up the confusion I had. I modified the tests now to test the expected behavior!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.4%. Comparing base (fb74565) to head (aef1e3d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2724     +/-   ##
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+ Coverage   79.3%   79.4%   +0.1%     
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  Files        123     123             
  Lines      22682   22737     +55     
=======================================
+ Hits       17992   18059     +67     
+ Misses      4690    4678     -12     

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