Use a consistent key for the current span in the context

[pyohannes]

The Extract call on the W3C propagator used to store the extracted span under the key current-span in the context, whereas on other places we rely on the current span being stored under the key span_key. This discrepancy will break context propagation.

This PR unifies this to use a constant kSpanKey in all relevant places and also adds tests.

[codecov]

Codecov Report

Merging #480 (3e4e3c3) into master (ab7a8a2) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #480      +/-   ##
==========================================
- Coverage   94.43%   94.37%   -0.06%     
==========================================
  Files         187      189       +2     
  Lines        8244     8410     +166     
==========================================
+ Hits         7785     7937     +152     
- Misses        459      473      +14     

Impacted Files	Coverage Δ
api/include/opentelemetry/trace/span.h	100.00% <ø> (ø)
...entelemetry/trace/propagation/http_trace_context.h	86.79% <100.00%> (ø)
api/include/opentelemetry/trace/scope.h	100.00% <100.00%> (ø)
api/include/opentelemetry/trace/tracer.h	100.00% <100.00%> (ø)
...pi/test/trace/propagation/http_text_format_test.cc	98.30% <100.00%> (ø)
api/test/trace/scope_test.cc	100.00% <100.00%> (ø)
sdk/src/trace/tracer.cc	73.52% <100.00%> (ø)
api/include/opentelemetry/nostd/string_view.h	97.56% <0.00%> (+0.06%)	⬆️
... and 1 more