Add Span::prepare_for_async method and AbstractSpan trait.

[jmjoy]

Sometimes we need to create a span following by the last active span instead of as the a child span, like in https://github.com/apache/skywalking-php/blob/eabb210b42ff652b8e34d8d99a933b5489b80397/src/plugin/plugin_curl.rs#LL276C9-L276C9

1. Add Span::prepare_for_async mehtod, which will return the AsyncSpan object, when it dropped, it will auto finished and notify TracingContext.
2. Add TracingContext::wait method, which will block to wait all AsyncSpan finished.
3. Add AbstractSpan trait, Span and AsyncSpan all implement it.

[codecov-commenter]

Codecov Report

Merging #58 (d702feb) into master (632766f) will decrease coverage by 1.01%.
The diff coverage is 6.73%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
- Coverage   17.69%   16.69%   -1.01%     
==========================================
  Files          17       18       +1     
  Lines         565      629      +64     
==========================================
+ Hits          100      105       +5     
- Misses        465      524      +59     

Impacted Files	Coverage Δ
src/logging/record.rs	0.00% <ø> (ø)
src/trace/span.rs	0.00% <0.00%> (ø)
src/trace/trace_context.rs	24.07% <9.09%> (-0.93%)	⬇️
src/common/wait_group.rs	15.38% <15.38%> (ø)

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[wu-sheng]

By following, you mean sharing the same parent span with the current active span? Two active spans created and running?

[jmjoy]

By following, you mean sharing the same parent span with the current active span? Two active spans created and running?

Yes, like curl_multi_*, many HTTP requests are requested simultaneously in a multi-threaded manner, so they should belong to the same parent. Is there a better way?

[wu-sheng]

If they run concurrently, usually we prefer span created and change into async mode(not be active span anymore)

https://skywalking.apache.org/docs/skywalking-java/next/en/setup/service-agent/java-agent/java-plugin-development-guide/#async-span-apis

This is Java dev doc. Notice, Java trace segment is in one thread only, so, when thread crossing happens, you need to go for new segment. But then the span duration can't represent the HTTP call anymore. So, we need async span to create in thread A, but finish in thread B. This concept should fit your scenarios as well.

[wu-sheng]

follow up span was introduced by Opentracing, but very confusing. So, OTEL doesn't mention that, SkyWalking never have this concept too.

[wu-sheng]

Please update doc about new APIs.

[jmjoy]

Please update doc about new APIs.

The documentation for this part of the API has already been mentioned in the document comments (generated in docs.rs), and mentioning it on readme will make it very large?

[wu-sheng]

Yes, such as how to typically use these new APIs.