Add span.TracerProvider()

[Aneurysm9]

This PR adds a TracerProvider() method to the trace.Span interface. This method allows instrumentation authors to obtain a TracerProvider without pre-configuration by extracting a Span from a Context and utilizing the TracerProvider that was used to construct the extracted Span.

[codecov]

Codecov Report

Merging #2009 (c3d53d4) into main (d020e1a) will increase coverage by 0.0%.
The diff coverage is 91.3%.

@@          Coverage Diff          @@
##            main   #2009   +/-   ##
=====================================
  Coverage   73.0%   73.0%           
=====================================
  Files        171     171           
  Lines       8675    8687   +12     
=====================================
+ Hits        6338    6348   +10     
- Misses      2068    2070    +2     
  Partials     269     269           

Impacted Files	Coverage Δ
bridge/opentracing/internal/mock.go	68.7% <0.0%> (-0.7%)	⬇️
trace/trace.go	98.2% <ø> (ø)
trace/noop.go	50.0% <75.0%> (+3.8%)	⬆️
internal/global/trace.go	84.2% <100.0%> (+0.4%)	⬆️
oteltest/harness.go	84.6% <100.0%> (+0.4%)	⬆️
oteltest/provider.go	100.0% <100.0%> (ø)
oteltest/span.go	100.0% <100.0%> (ø)
oteltest/tracer.go	89.6% <100.0%> (ø)
sdk/trace/span.go	88.9% <100.0%> (+<0.1%)	⬆️