Support correlation

[mrproliu]

Support correlation custom data to tracing context.

[codecov-io]

Codecov Report

Merging #91 (2a8ae15) into master (f4f82c8) will decrease coverage by 0.67%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
- Coverage   67.02%   66.34%   -0.68%     
==========================================
  Files          13       14       +1     
  Lines         561      621      +60     
==========================================
+ Hits          376      412      +36     
- Misses        151      170      +19     
- Partials       34       39       +5     

Impacted Files	Coverage Δ
propagation/propagation.go	50.53% <37.77%> (-11.97%)	⬇️
correlation.go	85.71% <85.71%> (ø)
plugins/http/client.go	48.78% <100.00%> (ø)
plugins/http/server.go	50.00% <100.00%> (ø)
segment.go	91.01% <100.00%> (+0.31%)	⬆️
trace.go	80.00% <100.00%> (+1.64%)	⬆️
trace_opts.go	53.33% <0.00%> (+26.66%)	⬆️

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[wu-sheng]

This PR breaks the existing propagation APIs, but it is reasonable. @kezhenxu94 @arugal @hanahmily please review the codes detail.

[wu-sheng]

We need to make the API stable before the 1.0 release.

[wu-sheng]

@mrproliu I think this PR would break all plugins in https://github.com/SkyAPM/go2sky-plugins as most of them are going context propagation. You need to update those after this merges.

[mrproliu]

@mrproliu I think this PR would break all plugins in https://github.com/SkyAPM/go2sky-plugins as most of them are going context propagation. You need to update those after this merges.

Of course. After this merge, I will update the plugins.

[wu-sheng]

@mrproliu Please update the plugins.