Support adding links to span

[lalitb]

Closing old PR #342 , as this was rework. Review comments from old PR have been incorporated here.

[codecov]

Codecov Report

Merging #351 (46ddd0c) into master (c504cfb) will increase coverage by 0.02%.
The diff coverage is 96.84%.

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
+ Coverage   94.98%   95.00%   +0.02%     
==========================================
  Files         162      164       +2     
  Lines        6997     7086      +89     
==========================================
+ Hits         6646     6732      +86     
- Misses        351      354       +3     

Impacted Files	Coverage Δ
api/include/opentelemetry/trace/noop.h	86.20% <ø> (ø)
sdk/include/opentelemetry/sdk/trace/tracer.h	100.00% <ø> (ø)
sdk/src/trace/span.h	100.00% <ø> (ø)
...pentelemetry/trace/span_context_kv_iterable_view.h	84.21% <84.21%> (ø)
...ude/opentelemetry/trace/span_context_kv_iterable.h	100.00% <100.00%> (ø)
api/include/opentelemetry/trace/tracer.h	100.00% <100.00%> (ø)
api/test/trace/noop_test.cc	100.00% <100.00%> (ø)
sdk/src/trace/span.cc	89.01% <100.00%> (+0.63%)	⬆️
sdk/src/trace/tracer.cc	73.52% <100.00%> (ø)
sdk/test/trace/tracer_test.cc	100.00% <100.00%> (ø)
... and 2 more

[pyohannes]

Thanks for diving into template metaprogramming. This looks good to me.

[lalitb]

Thanks for diving into template metaprogramming. This looks good to me.

Thanks for your guidance and reviewing it.

[lalitb]

@reyang This should be good to merge once all the checks are completed.