Add span flags to OTLP exported data

[dmathieu]

This adds the new span flags into the OTLP exported data.
See open-telemetry/opentelemetry-proto#503

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.3%. Comparing base (e518c7e) to head (25e9c88).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5194   +/-   ##
=====================================
  Coverage   84.3%   84.3%           
=====================================
  Files        258     258           
  Lines      16966   16974    +8     
=====================================
+ Hits       14305   14313    +8     
  Misses      2362    2362           
  Partials     299     299           

Files	Coverage Δ
...ers/otlp/otlptrace/internal/tracetransform/span.go	96.7% <100.0%> (+0.2%)	⬆️

[AlexanderWert]

Two things:

1. The is remote parent falgs need also to be set for the SpanLinks flags.
2. We need to retrieve the information about isRemote from the parent's SpanContext (see comment below)

[estolfo]

after Alex's comments are addressed, LGTM