Propagate trace and parent span ids

[nadiaciobanu]

Now that the Span class contains a SpanContext instance, we can propagate trace and parent span ids using SpanContext. When the tracer creates a new Span, get the SpanContext of the current span from the runtime context and pass it to the Span constructor. In the Span constructor, create a new SpanContext and use the parent span context to set the trace and parent span id.

I exported to GCP using the OTLP example (example/otlp) and verified that the trace and parent span ids propagate properly:

[codecov]

Codecov Report

Merging #305 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #305      +/-   ##
==========================================
+ Coverage   94.55%   94.60%   +0.04%     
==========================================
  Files         146      146              
  Lines        6610     6633      +23     
==========================================
+ Hits         6250     6275      +25     
+ Misses        360      358       -2     

Impacted Files	Coverage Δ
sdk/src/trace/span.cc	84.33% <100.00%> (+3.56%)	⬆️
sdk/src/trace/tracer.cc	73.52% <100.00%> (+4.56%)	⬆️
sdk/test/trace/tracer_test.cc	99.09% <100.00%> (+0.01%)	⬆️
...pi/include/opentelemetry/context/runtime_context.h	95.83% <0.00%> (-4.17%)	⬇️
api/include/opentelemetry/trace/span_context.h	100.00% <0.00%> (ø)
sdk/src/trace/span.h	100.00% <0.00%> (+100.00%)	⬆️

[IlyaKobelevskiy]

LGTM

[reyang]

LGTM.

[nadiaciobanu]

@reyang Ready to merge :)