Change Span to refer to SpanStack, rather than TracingContext.

[jmjoy]

In order to improve the performance of TracingContext, reduce the usage of Arc and Lock.

[codecov-commenter]

Codecov Report

Merging #39 (508b2c7) into master (27959a5) will increase coverage by 0.50%.
The diff coverage is 20.22%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
+ Coverage   43.11%   43.62%   +0.50%     
==========================================
  Files          14       14              
  Lines         385      392       +7     
==========================================
+ Hits          166      171       +5     
- Misses        219      221       +2     

Impacted Files	Coverage Δ
src/context/span.rs	0.00% <0.00%> (ø)
src/context/tracer.rs	57.29% <ø> (ø)
tests/trace_context.rs	0.00% <ø> (ø)
src/context/trace_context.rs	23.37% <25.35%> (+2.28%)	⬆️

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[wu-sheng]

Why is a segment link? Segment reference should be inside a span, and it should be multiple references.

[jmjoy]

This field existed before I was involved in this project. 😂

[wu-sheng]

Let's check why it exists. and try to remove it?

[jmjoy]

Ok, actually I also think this field is not very right.

[jmjoy]

Remove TracingContext segment_link field, and add TracingContext create_entry_span_with_propagation method.

[wu-sheng]

TracingContext create_entry_span_with_propagation method.

This is good.

A tip, trace ID could be generated as a new trace, and then do Inject later to override the pre generated, when injecting context happens after span creation. This is not necessary, but we do this in Java.

[wu-sheng]

LGTM. Feel free to merge, notice, please clear the commits in the merge message.

[jmjoy]

TracingContext create_entry_span_with_propagation method.

This is good.

A tip, trace ID could be generated as a new trace, and then do Inject later to override the pre generated, when injecting context happens after span creation. This is not necessary, but we do this in Java.

OK, I'll take a look later.